Neurite growth acceleration of adult Dorsal Root Ganglion neurons illuminated by low‐level Light Emitting Diode light at 645 nm

The effect of a 645 nm Light Emitting Diode (LED) light irradiation on the neurite growth velocity of adult Dorsal Root Ganglion (DRG) neurons with peripheral axon injury 4–10 days before plating and without previous injury was investigated. The real amount of light reaching the neurons was calculated by taking into account the optical characteristics of the light source and of media in the light path. The knowledge of these parameters is essential to be able to compare results of the literature and a way to reduce inconsistencies. We found that 4 min irradiation of a mean irradiance of 11.3 mW/cm² (corresponding to an actual irradiance reaching the neurons of 83 mW/cm²) induced a 1.6‐fold neurite growth acceleration on non‐injured neurons and on axotomized neurons. Although the axotomized neurons were naturally already in a rapid regeneration process, an enhancement was found to occur while irradiating with the LED light, which may be promising for therapy applications.

Dorsal Root Ganglion neurons ( A ) without previous injury and ( B ) subjected to a conditioning injury.     

Which wavelength is optimal for transcranial low‐level laser stimulation?

One of the challenges in transcranial low‐level laser therapy (LLLT) is to optimally choose illumination parameters, such as wavelength. However, there is sparse study on the wavelengths comparison especially on human transcranial LLLT. Here, we employed Monte Carlo modeling and visible human phantom to compute the penetrated photon fluence distribution within cerebral cortex. By comparing the fluence distribution, penetration depth and the intensity of laser‐tissue‐interaction within brain among all candidate wavelengths, we found that 660, 810 nm performed much better than 980, 1064 nm with much stronger, deeper and wider photon penetration into cerebral tissue; 660 nm was shown to be the best and slightly better than 810 nm. Our computational finding was in a surprising accordance with previous LLLT‐neurobehavioral studies on mice. This study not only offered quantitative comparison among wavelengths in the effect of LLLT light penetration effectiveness but also anticipated a delightful possibility of online, precise and visible optimization of LLLT illumination parameters.      

Safety of light emitting diode‐red light on human skin: Two randomized controlled trials

Therapeutic applications of light emitting diode‐red light (LED‐RL) are expanding, yet data on its clinical effects are lacking. Our goal was to evaluate the safety of high fluence LED‐RL (≥160 J/cm²). In two phase I, single‐blind, dose escalation, randomized controlled trials, healthy subjects received LED‐RL or mock irradiation to the forearm thrice weekly for 3 weeks at fluences of 160‐640 J/cm² for all skin types (STARS 1, n = 60) and at 480‐640 J/cm² for non‐Hispanic Caucasians (STARS 2, n = 55). The primary outcome was the incidence of adverse events (AEs). The maximum tolerated dose was the highest fluence that did not elicit predefined AEs. Dose‐limiting AEs, including blistering and prolonged erythema, occurred at 480 J/cm² in STARS 1 (n = 1) and 640 J/cm² in STARS 2 (n = 2). AEs of transient erythema and hyperpigmentation were mild. No serious AEs occurred. We determined that LED‐RL is safe up to 320 J/cm² for skin of color and 480 J/cm² for non‐Hispanic Caucasian individuals. LED‐RL may exert differential cutaneous effects depending on race and ethnicity, with darker skin being more photosensitive. These findings may guide future studies to evaluate the efficacy of LED‐RL for the treatment of various diseases.     

A compact high‐speed full‐field optical coherence microscope for high‐resolution in vivo skin imaging

A compact high‐speed full‐field optical coherence microscope has been developed for high‐resolution in vivo imaging of biological tissues. The interferometer, in the Linnik configuration, has a size of 11 × 11 × 5 cm³ and a weight of 210 g. Full‐field illumination with low‐coherence light is achieved with a high‐brightness broadband light‐emitting diode. High‐speed full‐field detection is achieved by using part of the image sensor of a high‐dynamic range CMOS camera. En face tomographic images are acquired at a rate of 50 Hz, with an integration time of 0.9 ms. The image spatial resolution is 0.9 μm × 1.2 μm (axial × transverse), over a field of view of 245 × 245 μm². Images of human skin, revealing in‐depth cellular‐level structures, were obtained in vivo and in real‐time without the need for stabilization of the subject. The system can image larger fields, up to 1 × 1 mm², but at a reduced depth.      

Longevity and quality of cut 'Master' carnation and 'Red Sandra' rose flowers as affected by red light

Cut flowering stems (45 cm long) of carnation (cv. Master) and rose (cv. Red Sandra) were placed in test tubes containing distilled water with or without commercial preservative (20 ml l^–1), under fluorescent tubes of 50 ± 10 mol m^–2 s^–1 Photosynthetic photon flux (PPF). A mixed radiation from fluorescent tubes with red light provided by light emitting diodes (LEDs) and monochromic red light of low 50 or high 90 ± 10 mol m^–2 s^–1 was also tested in the absence of preservative solution. Both red light with high PPF and the mixed radiation under low PPF extended the vase life of cut carnations, and flower freshness could be maintained for 10.9 days compared with a water/fluorescent light control. In cut rose, the treatment containing the preservative solution under fluorescent tubes alone and in red alone, regardless of light intensity, prolonged vase life for 4.6 and 4.2 days longer than the control, respectively. Treatment with red LEDs plus high PPF resulted in complete petal opening in carnation flowers.     

Multi‐wavelength light emitting diode‐based disposable optrode array for in vivo optogenetic modulation

We present a light emitting diode (LED)‐based optical waveguide array that can optogenetically modulate genetically targeted neurons in the brain. The reusable part of the system consists of control electronics and conventional multi‐wavelength LED. The disposable part comprises optical fibers assembled with microlens array fabricated on a silicon die. Both parts can be easily assembled and separated by snap fit structure. Measured light intensity is 3.35 mW/mm² at 469 nm and 0.29 mW/mm² at 590 nm when the applied current is 80 mA. In all the tested conditions, the light‐induced temperature rise is under 0.5°C and over 90% of the relative light intensity is maintained at 2 mm‐distance from the fiber tips. We further tested the efficiency of the optical array in vivo at 469 nm. When the optical array delivers light stimulation on to the visual cortex of a mouse expressing channelrhodopsin‐2, the neural activity is significantly increased. The light‐driven neural activity is successfully transformed into a percept of the mouse, showing significant learning of the task detecting the cortical stimulation. Our results demonstrate that the proposed optical array interfaces well with the neural circuits in vivo and the system is applicable to guide animal behaviors.      

The effects of light intensity and spectral quality on growth and shoot initiation in tobacco callus

The effects of eight different narrow band-emitting fluorescent lamps (371-750 nm) and four commercial broad band-emitting fluorescent sources upon growth and shoot initiation in tobacco callus (Nicotiana tabacum var. Wisconsin 38) have been characterized. Wavelength and intensity are equally important parameters in determining morphogenic changes. Near ultraviolet light (371 nm) was found to stimulate (0.024 mw/cm²) or inhibit (above 0.15 mw/cm²) callus growth and shoot initiation, depending on the light intensity. Stimulation of growth and shoot production occurs also in blue light region, but at higher intensity than in the near ultraviolet. Red and far red light (up to 1.7 mw/cm²) do not appear to affect callus growth or stimulate shoot initiation. The enhancement of callus growth and the stimulation of shoot initiation are controlled by the same near ultraviolet-absorbing photoreceptor system present in a small enough concentration that it cannot be recognized in the absorption spectrum of the intact tissue. Carotenoids, porphyrins, and phytochrome associated with the high irradiance response do not appear to qualify as the photoreceptor. Flavonoids are possible candidates. Radiation emitted by fluorescent lamps outside the near visible region was determined, and we concluded that energy levels were not sufficient to affect the reported results. The spectral output of several commercial lamps in the visible and near visible regions is such that there could be different effects on growth and development of tissue cultures.     

Light‐emitting diode irradiation using 660 nm promotes human fibroblast HSP90 expression and changes cellular activity and morphology

We evaluated changes in cell viability and morphology in response to low‐level light irradiation and underlying variations in the levels of heat shock proteins (HSPs). Human fibroblasts were irradiated with a light‐emitting diode (LED) array at 660 nm (50 mW for 15, 30, and 60 minutes). Cell viability and morphological changes were evaluated via epifluorescence analysis; we also assessed cell viability and length changes. The expression levels of adenosine triphosphate (ATP) and various HSPs (HSP27, 60, 70, and 90) were analyzed by immunohistochemical staining, Western blotting and microarray analysis. After LED irradiation, cellular viability and morphology changed. Of the several HSPs analyzed, the HSP90 level increased significantly, suggesting that this protein played roles in the morphological and cellular changes. Thus, low‐level irradiation triggered cellular changes mediated by increased HSP90 expression; this may explain why skin irradiation enhances wound‐healing.     

An irreversible red-light-induced growth response in Avena

Summary The inhibition of the growth rate of the first internode of Avena by red light occurs in three steps. The first step reduces elongation by ca. 15%. It is produced and saturated by 10^-3 to 10^-1 W sec cm^-2 at =660 nm and is irreversible by far-red irradiation. All wavelengths between 400 and 800 nm produce and saturate this step. The second step, produced by red light quantities between ca. 10 and 10⁴ W sec cm^-2 reduces elongation to ca. 50% of the maximal; it is not produced by far-red light but far-red reverses completely this component of the effect of red light. The third step inhibits mesocotyl elongation to ca. 95% of the maximal. The effect of red light in this step depends on the duration of irradiation rather than on the total quantity of energy, and is not reversed by far-red irradiation. The three inhibition steps in the elongation of the mesocotyl are matched by three growth-promotion steps in the growth of the coleoptile, but the extent of the far-red-irreversible first step outweighs in this case by far the extent of the far-red-reversible second step.     

Superpulsed (Ga‐As, 904 nm) low‐level laser therapy (LLLT) attenuates inflammatory response and enhances healing of burn wounds

Low‐level laser therapy (LLLT) using superpulsed near‐infrared light can penetrate deeper in the injured tissue and could allow non‐pharmacological treatment for chronic wound healing. This study investigated the effects of superpulsed laser (Ga‐As 904 nm, 200 ns pulse width; 100 Hz; 0.7 mW mean output power; 0.4 mW/cm² average irradiance; 0.2 J/cm² total fluence) on the healing of burn wounds in rats, and further explored the probable associated mechanisms of action. Irradiated group exhibited enhanced DNA, total protein, hydroxyproline and hexosamine contents compared to the control and silver sulfadiazine (reference care) treated groups. LLLT exhibited decreased TNF‐α level and NF‐kB, and up‐regulated protein levels of VEGF, FGFR‐1, HSP‐60, HSP‐90, HIF‐1α and matrix metalloproteinases‐2 and 9 compared to the controls. In conclusion, LLLT using superpulsed 904 nm laser reduced the inflammatory response and was able to enhance cellular proliferation, collagen deposition and wound contraction in the repair process of burn wounds.

Photomicrographs showing no, absence inflammation and faster wound contraction in LLLT superpulsed (904 nm) laser treated burn wounds as compared to the non‐irradiated control and silver sulfadiazine (SSD) ointment (reference care) treated wounds     

Photobiomodulation alters matrix protein activity in stressed fibroblast cells in vitro

A balance is maintained between matrix synthesis and degradation, and a prolonged increase in matrix metalloproteinases (MMPs) affects healing. Photobiomodulation (PBM) speeds up healing and alters wound environment. The study aimed to determine changes in protein and gene expression of collagen type 1 (Col‐I), MMP‐3 and ‐9 and TIMP‐1 in fibroblasts irradiated at 660 or 830 nm. Commercially purchased human skin fibroblast cells were modeled into five groups namely, normal, normal wounded, diabetic wounded, hypoxic wounded and diabetic hypoxic wounded. Control cells were sham irradiated. Laser irradiation was conducted at 660 or 830 nm (108/or 94 mW, 9.1 cm², 420/or 483 s) with 5 J/cm². Forty‐eight hours post‐irradiation, protein expression of TIMP‐1, MMP‐3, ?9 and Col‐I was determined by flow cytometry and immunofluorescence, and gene expression by real‐time RT‐PCR. There was an increase in TIMP‐1 and Col‐I, and a decrease in MMP‐3 and ‐9, as well as an alteration in mRNA expression of MMP3, MMP9, TIMP1 and COL1A1 in irradiated cells. Due to the responsiveness of the diabetic hypoxic wounded model, the findings propose this model as appropriate for wound healing studies and suggest that PBM promotes the remodeling phase of wound healing by decreasing matrix degradation and upregulating synthesis.      

Light‐emitting diode therapy in exercise‐trained mice increases muscle performance,cytochrome c oxidase activity,ATP and cell proliferation

Light‐emitting diode therapy (LEDT) applied over the leg, gluteus and lower‐back muscles of mice using a LED cluster (630 nm and 850 nm, 80 mW/cm², 7.2 J/cm²) increased muscle performance (repetitive climbing of a ladder carrying a water‐filled tube attached to the tail), ATP and mitochondrial metabolism; oxidative stress and proliferative myocyte markers in mice subjected to acute and progressive strength training. Six bi‐daily training sessions LEDT‐After and LEDT‐Before‐After regimens more than doubled muscle performance and increased ATP more than tenfold. The effectiveness of LEDT on improving muscle performance and recovery suggest applicability for high performance sports and in training programs.

Positioning of the mice and light‐emitting diode therapy (LEDT) applied on mouse legs, gluteus and lower‐back muscles without contact.     

Phytochrome Controlled Adhesion of Mung Bean Root Tips to Glass: A Detailed characterization of the Phenomenon

Various parameters of the Tanada effect (Proc. Natl. Acad. Sci. U.S. 59: 376–380. 1968) have been defined. This phenomenon, in which root tips of Phaseolus aureus L. adhere to a negatively charged glass surface when they are irradiated with 660 nm (red) light and release under 730 nm (far-red) light, has been characterized as follows. Secondary roots, whether etiolated or light grown exhibit photoreversible adhesion. Primary roots do not. Tips from 6–8 mm secondary roots exhibit the best response to red light, whereas tips from 3 mm roots respond best to far-red light. Red light saturetes the adhesion system at about 50 μ W/cm²xnm and far-red light, release system at about 150 ü W/cm² xnm. The adhesion effect begins to show escape from far-red reversibility within 60–90 seconds, an observation quite different from other “typical” long term de- etiolation effects. In addition, root tips irradiated with red light begin to release spontaneously in the dark after 10 min, and have nearly completed release after 50 min. Tips irradiated with continuous red light show gradual release after 15 minutes of exposure. Whether these data indicate an extremely rapid dark reversion of P_fr to P_r or decay of P_fr under continuous red light is not known at this time. In order to study tip adhesion and release, the glass beaker surface may be negatively charged with thiocyanate (SCN^-), nitrate (NO₃^-), sulfate (SO₄^2-), chloride (Cl^-), phosphate (PO₄^3-), citrate (C₆H₅O₇^3-), oxalate (C₂O₄^2-) or glutamine (C₅H₈NO₄^-). Benzoate (C₇H₅O₂^-) and acetate (CH₃COO^-) were found to be relatively ineffective for red light adhesion, however when citrate and oxalate were used release was inhibited. This was apparently due to a chelation of Ca²⁺since release began immediately as excess Ca⁺² was added to the bathing solution. Substitution of GTP, ITP, UTP, or CTP for ATP resulted in only 20 to 40% adhesion and release for GTP, ITP and UTP, CTP showed normal adhesion kinetics under red light but very slow release kinetics under far-red light. The effects of red and far-red light in the numbers of secondary roots are that red light inhibits root initiation while far-red light partially reverses the red light effect.     

Photodynamic therapy associating Photogem® and blue LED on L929 and MDPC‐23 cell culture

To evaluate the cytotoxicity of PDT (photodynamic therapy) with Photogem® associated to blue LED (light‐emitting diode) on L929 and MDPC‐23 cell cultures, 30000 cells/cm² were seeded in 24‐well plates for 48 h, incubated with Photogem® (10, 25 or 50 mg/l) and irradiated with an LED source (460±3 nm; 22 mW/cm²) at two energy densities (25.5 or 37.5 J/cm²). Cell metabolism was evaluated by the MTT (methyltetrazolium) assay (Dunnet's post hoc tests) and cell morphology by SEM (scanning electron microscopy). Flow cytometry analysed the type of PDT‐induced cell death as well and estimated intracellular production of ROS (reactive oxygen species). There was a statistically significant decrease of mitochondrial activity (90% to 97%) for all Photogem® concentrations associated to blue LED, regardless of irradiation time. It was also demonstrated that the mitochondrial activity was not recovered after 12 or 24 h, characterizing irreversible cell damage. PDT‐treated cells presented an altered morphology with ill‐defined limits. In both cell lines, there was a predominance of necrotic cell death and the presence of Photogem® or irradiation increased the intracellular levels of ROS. PDT caused severe toxic effects in normal cell culture, characterized by the reduction of the mitochondrial activity, morphological alterations and induction of necrotic cell death.     

Photobiomodulation induces antinociception,recovers structural aspects and regulates mitochondrial homeostasis in peripheral nerve of diabetic mice

Diabetic peripheral neuropathy (DPN) is a nervous disorder caused by diabetes mellitus, affecting about 50% of patients in clinical medicine. Chronic pain is one of the major and most unpleasant symptoms developed by those patients, and conventional available treatments for the neuropathy, including the associated pain, are still unsatisfactory and benefit only a small number of patients. Photobiomodulation (PBM) has been gaining clinical acceptance once it is able to promote early nerve regeneration resulting in significant improvement in peripheral nerves disabilities. In this work, the effects of PBM (660 nm, 30 mW, 1.6 J/cm², 0.28 cm², 15 s in a continuous frequency) on treating DPN‐induced pain and nerve damage were evaluated in an experimental model of diabetic‐neuropathy induced by streptozotocin in mice. PBM‐induced antinociception in neuropathic‐pain mice was dependent on central opioids release. After 21 consecutive applications, PBM increased nerve growth factor levels and induced structural recovery increasing mitochondrial content and regulating Parkin in the sciatic nerve of DPN‐mice. Taking together, these data provide new insights into the mechanisms involved in the effects of PBM‐therapy emphasizing its therapeutic potential in the treatment of DPN.      

A short red light pulse during dark phase of LD-cycle perturbs the hamster's circadian clock

In this study we investigated the influence of red light, which naturally occurs during dawn and dusk, on locomotor activity and body temperature rhythms of Djungarian hamsters (Phodopus sungarus). A single weak red light pulse given 2 h before regular lights on had acute as well as long-term effects persisting for several days following exposure. The hamsters immediately stopped their locomotor activity, accompanied by a drop in body temperature. In the following undisturbed nights (LD 168) the nocturnal activity stopped earlier than usual. This lasting effect of the light pulse was more pronounced than the acute effect. The activity phase compressed gradually during 3 to 5 days after the light pulse was administered while time of activity onset was almost unaffected. It took 6 to 11 days for complete recovery of the original activity phase. The maximal activity compression and the recovery period depended on the duration of the single red light pulse and its intensity. Red light pulses of 15 min duration were about twice effective as 1 min pulses; and the effect of a red light pulse of 130 mW/m² was about 1.5 times stronger than a 30 mW/m² red light pulse. The maximal value of activity phase compression reached in this experiment was 2.5+0.2 h with a recovery period of 11.1±0.3 days following a given red light pulse of 90 mW/m² and 15 min. The morning oscillator seems to be persistently affected. This indicates a very high photosensitivity of the Djungarian hamster's circadian system to red light.Abbreviations T _b body temperature - DD constant darkness - LD light:dark cycle - LL constant light - duration of activity phase - CT circadian time - PRC phase response curve - SCN suprachiasmatic nuclei     

Luminescence properties of red‐emission Mg4Nb2O9:Eu3+ phosphor

Red‐emitting Mg₄Nb₂O₉:Eu³⁺ phosphor is synthesized via a solid‐state reaction method in air, and its crystal structure and luminescence are investigated. The phosphor can be excited efficiently by ~ 395 nm light, coupled well with a ~ 395 nm near‐ultraviolet chip and emits red light at ~ 613 nm with sharp spectra due to ⁵D₀ → ⁷ F₂ transition of the Eu³⁺ ion. Mg₄Nb₂O₉:Eu³⁺ phosphor sintered at 1350 ºC shows Commission international de I'Eclairage (CIE) chromaticity coordinates of x = 0.6354, y = 0.3592, and is a potential red‐emitting phosphor candidate for white light‐emitting diodes (W‐LEDs) under ~ 395 nm near‐ultraviolet LED chip excitation. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of irradiance on growth,photosynthesis, and water use efficiency of seedlings of the chaparral shrub,Ceanothus megacarpus

Summary The effects of irradiance during growth on biomass allocation, growth rates, leaf chlorophyll and protein contents, and on gas exchange responses to irradiance and CO₂ partial pressures of the evergreen, sclerophyllous, chaparral shrub, Ceanothus megacarpus were determined. Plants were grown at 4 irradiances for the growth experiments, 8, 17, 25, 41 nE cm^-2 sec^-1, and at 2 irradiances, 9 and 50 nE cm^-2 sec^-1, for the other comparisons.At higher irradiances root/shoot ratios were somewhat greater and specific leaf weights were much greater, while leaf area ratios were much lower and leaf weight ratios were slightly lower than at lower irradiances. Relative growth rates increased with increasing irradiance up to 25 nE cm^-2 sec^-1 and then leveled off, while unit leaf area rates increased steeply and unit leaf weight rates increased more gradually up to the highest growth irradiance.Leaves grown at 9 nE cm^-2 sec^-1 had less total chlorophyll per unit leaf area and more per unit leaf weight than those grown at 50 nE cm^-2 sec^-1. In a reverse of what is commonly found, low irradiance grown leaves had significantly higher chlorophyll a/b than high irradiance grown leaves. High irradiance grown leaves had much more total soluble protein per unit leaf area and per unit dry weight, and they had much higher soluble protein/chlorophyll than low irradiance grown leaves.High irradiance grown leaves had higher rates of respiration in very dim light, required higher irradiances for photosynthetic saturation and had higher irradiance saturated rates of photosynthesis than low irradiance grown leaves. CO₂ compensation irradiances for leaves of both treatments were very low, <5 nE cm^-2 sec^-1. Leaves grown under low and those grown under high irradiances reached 95% of their saturated photosynthetic rates at 65 and 85 nE cm^-2 sec^-1, respectively. Irradiance saturated rates of photosynthesis were high compared to other chaparral shrubs, 1.3 for low and 1.9 nmol CO₂ cm^-2 sec^-1 for high irradiance grown leaves. A very unusual finding was that leaf conductances to H₂O were significantly lower in the high irradiance grown leaves than in the low irradiance grown leaves. This, plus the differences in photosynthetic rates, resulted in higher water use efficiencies by the high irradiance grown leaves. High irradiance grown leaves had higher rates of photosynthesis at any particular intercellular CO₂ partial pressure and also responded more steeply to increasing CO₂ partial pressure than did low irradiance grown leaves. Leaves from both treatments showed reduced photosynthetic capability after being subjected to low CO₂ partial pressures (100 bars) under high irradiances. This treatment was more detrimental to leaves grown under low irradiances.The ecological implications of these findings are discussed in terms of chaparral shrub community structure. We suggest that light availability may be an important determinant of chaparral community structure through its effects on water use efficiencies rather than on net carbon gain.     

Comparative analysis of two laser wavelengths in the stimulation of acupuncture points for analgesic effects in an animal model

This study compares the effectiveness of two laser wavelengths for stimulating acupoints in an experimental model of acute postoperative pain. Forty-five Wistar rats were randomly assigned to receive treatment on their left hind paw, contralateral to a surgical procedure. Laser treatments were performed with Green Laser—GL (532 nm, 70 mW and 7 J/cm² of energy), Red Laser—RL (660 nm, 100 mW and 7 J/cm² of energy), or with Laser Off—LO. After each application, the animals were evaluated with a Von Frey analgesiometer to check for painful sensitivity on their right (with surgery) and left (without surgery) hind paws. Neuropeptides and cytokine levels in the incision site tissue of the right paw were measured by ELISA after 1, 6 and 24 hours. It was possible to observe that, in this pain model, both lasers promoted analgesia and that the GL altered the levels of TNF-α and IL-1β.     

IMPORTANCE OF MACRO‐ VERSUS MICROSTRUCTURE IN MODULATING LIGHT LEVELS INSIDE CORAL COLONIES1

Adjusting the light exposure and capture of their symbiotic photosynthetic dinoflagellates (genus Symbiodinium Freud.) is central to the success of reef‐building corals (order Scleractinia) across high spatio‐temporal variation in the light environment of coral reefs. We tested the hypothesis that optical properties of tissues in some coral species can provide light management at the tissue scale comparable to light modulation by colony architecture in other species. We compared within‐tissue scalar irradiance in two coral species from the same light habitat but with contrasting colony growth forms: branching Stylophora pistillata and massive Lobophyllia corymbosa. Scalar irradiance at the level of the symbionts (2 mm into the coral tissues) were <10% of ambient irradiance and nearly identical for the two species, despite substantially different light environments at the tissue surface. In S. pistillata, light attenuation (90% relative to ambient) was observed predominantly at the colony level as a result of branch‐to‐branch self‐shading, while in L. corymbosa, near‐complete light attenuation (97% relative to ambient) was occurring due to tissue optical properties. The latter could be explained partly by differences in photosynthetic pigment content in the symbiont cells and pigmentation in the coral host tissue. Our results demonstrate that different strategies of light modulation at colony, polyp, and cellular levels by contrasting morphologies are equally effective in achieving favorable irradiances at the level of coral photosymbionts.