International Lighting in Controlled Environments Workshop

  T.W.Tibbitts (editor)  1994   NASA-CP-95-3309               home | contents

OCULAR HAZARDS OF LIGHT

David H. Sliney, Ph.D.

BACKGROUND

The eye is protected against bright light by the natural aversion response to viewing bright light sources. The aversion response normally protects the sun against injury from viewing bright light sources such as the sun, arc lamps and welding arcs, since this aversion limits the duration of exposure to a fraction of a second (about 0.25 s).

There are at least five separate types of hazards to the eye and skin from optical sources:1

  1. Ultraviolet photochemical injury to the skin (erythema and carcinogenic effects), and to the cornea (photokeratitis) and lens (cataract) of the eye (180 nm to 400 nm).

  2. Thermal injury to the retina of the eye (400 nm to 1400 nm)

  3. Blue-light photochemical injury to the retina of the eye (principally 400 nm to 550 nm; unless aphakic, 310 to 550 nm)2

  4. Near-infrared thermal hazards to the lens (approximately 800 nm to 3000 nm).

  5. Thermal injury (burns) of the skin (approximately 400 nm to l mm) and of the cornea of the eye (approximately 1400 nm to 1 mm).

The principal retinal hazard resulting from viewing bright light sources is photoretinitis, e.g., solar retinitis with an accompanying scotoma which results from staring at the sun. Solar retinitis was once referred to as "eclipse blindness" and associated "retinal burn." Only in recent years has it become clear that photoretinitis results from a photochemical injury mechanism following exposure of the retina to shorter wavelengths in the visible spectrum, i.e., violet and blue light. Prior to conclusive animal experiments at that time (Ham, Mueller and Sliney, 1976), it was thought to be a thermal injury mechanism. However, it has been shown conclusively that an intense exposure to short-wavelength light (hereafter referred to as "blue light") can cause retinal injury.

The product of the dose-rate and the exposure duration always must result in the same exposure dose (in joules-per-square centimeter at the retina) to produce a threshold injury. Blue-light retinal injury (photoretinitis) can result from viewing either an extremely bright light for a short time, or a less bright light for longer exposure periods. This characteristic of photochemical injury mechanisms is termed reciprocity and helps to distinguish these effects from thermal burns, where heat conduction requires a very intense exposure within seconds to cause a retinal coagulation; otherwise, surrounding tissue conducts the heat away from the retinal image. Injury thresholds for acute injury in experimental animals for both corneal and retinal effects have been corroborated for the human eye from accident data. Occupational safety limits for exposure to UVR and bright light are based upon this knowledge. As with any photochemical injury mechanism, one must consider the action spectrum, which describes the relative effectiveness of different wavelengths in causing a photobiological effect. The action spectrum for photochemical retinal injury peaks at approximately 440 nm.

CALCULATING RETINAL EXPOSURE

From knowledge of the optical parameters of the human eye and from radiometric parameters of a light source, it is possible to calculate irradiances (dose rates) at the retina. Exposure of the anterior structures of the human eye to ultraviolet radiation (UVR) may also be of interest; and the relative position of the light source and the degree of lid closure can greatly affect the proper calculation of this ultraviolet exposure dose. For ultraviolet and short-wavelength light exposures, the spectral distribution of the light source can also be important.

Quantities and units

Two sets of light-measurement quantities and units are useful in defining light exposure of the retina: radiometric and photometric. Radiometric quantities such as radiance--used to describe the "brightness" of a source [in W/cm2× sr] and irradiance--used to describe the irradiance level on a surface [in W/cm2] are particularly useful for hazard analysis. Radiance and luminance are particularly valuable because these quantities describe the source and do not vary with distance. Photometric quantities such as luminance (brightness in cd/cm2 as perceived by a human "standard observer") and illuminance in lux (the "light" falling on a surface) indicate light levels spectrally weighted by the standard photometric visibility curve which peaks at 550 nm for the human eye (Figure 1). To quantify a photochemical effect it is not sufficient to specify the number of photons-per-square-centimeter (photon flux) or the irradiance (W/cm2) since the efficiency of the effect will be highly dependent on wavelength. Generally, shorter-wavelength, higher-energy photons are more efficient.

Photometric quantities are hybrid quantities which are defined by an action spectrum for vision--a photochemically initiated process. Photometric quantities may not have much value in describing retinal effects other than vision or in research relating to neuroendocrine effects mediated by the visual system. Unfortunately, since the spectral distributions of different light sources vary widely, there is no simple conversion factor between photometric (either photopic or scotopic) and radiometric quantities. This conversion may vary from 15 to 50 lumens/watt (1m/W) for an incandescent source to about 100 1m/W for the sun or a xenon arc, to perhaps 300 to 400 lm/W for a fluorescent source (Sliney and Wolbarsht, 1980).1

HUMAN EXPOSURE LIMITS

A number of national and international groups have recommended occupational or public exposure limits (ELs) for optical radiation [i.e., ultraviolet (UV), light and infrared (IR) radiant energy]. Although most such groups have recommended ELs for UV and laser radiation, only one group has recommended ELs for visible radiation (i.e., light). This one group is well known in the field of occupational health--the American Conference of Governmental Hygienists (ACGIH). The ACGIH refers to its ELs as "Threshold Limit Values," or TLVs and these are issued yearly, so there is an opportunity for a yearly revision3-4. The current ACGIH TLV's for light (400 nm to 760 nm) have been largely unchanged for the last decade, although they have been on a tentative list for much of that time. They are based in large part on ocular injury data from animal studies and from data from human retinal injuries resulting from viewing the sun and welding arcs. The TLVs also have an underlying assumption that outdoor environmental exposures to visible radiant energy is normally not hazardous to the eye except in very unusual environments such as snow fields and deserts.

On the international scene there are currently no limits for optical radiation except for the special case of laser radiation. The International Non-ionizing Radiation Committee (INIRC) of the International Radiation Protection Association (IRPA) published Guidelines on Limits of Exposure to Laser Radiation in 19855 and revised them in 1988. INIRC guidelines are developed through collaboration with the World Health Organization (WHO) by jointly publishing criteria documents which provide the scientific data base for the exposure limits6.

THE ACGIH THRESHOLD LIMIT VALUES

Ultraviolet Radiation

The ACGIH TLV3 and the INIRC EL for exposure to the eye and skin to UVR is 3 mJ/cm2-effective, when the spectral irradiance Eλ at the eye or skin surface is mathematically weighted against the hazard sensitivity spectrum Sλ from 180 nm to 400 nm as follows:

Eeff = S Eλ × Sλ × Δ λ [1]

In addition to the above requirement, the ocular exposure is also limited to 1 J/cm2 for periods up to 1000 s (16.7 min) and to 1 mW/cm2 for greater periods. For this requirement, the total irradiance, E-uva, in the UV-A spectral region is summed from 315 nm to 400 nm:

E-uva = S Eλ × Δ λ [2]

where Eλ is the spectral irradiance in W/(cm2-nm).

The permissible exposure duration, tmax, in seconds, to UVR is calculated by:

tmax = (3 mJ/cm2) / Eeff (W/cm2) [3]

and if the UV-A irradiance exceeds the 8-hour criterion of 1 mW/cm2, the maximum exposure must also be less than:

tmax = (1 J/cm2) / EUVA (W/cm2) [4]

Retinal Thermal Hazards

The ACGIH TLV derived to protect the human retina from thermal injury requires the use of another spectral weighting function, Rλ.18 The TLV for the hazardous radiance is termed LHAZ, which is a function of the angular subtense α of the source (which is the light-source dimension DL divided by the viewing distance r to give the angle in radians) and the exposure duration t (in seconds):

LHAZ = 5 /α× t3/4 [in W/(cm2-sr] [5]

 

The spectral radiance Lλ of the source is weighted against the retinal hazard function Rλ and the resulting effective radiance must not exceed LHAZ:

Σ Lλ Rλ Δλ £ LHAZ (for t < 10 s) [6]

For small sources such as an optical fiber source, the closest distance at which the human eye can sharply focus upon a small object is about 10 cm. The value of 10 cm is an exceptionally small value for the near-point of accommodation for the human eye. At shorter distances the image of a light source would be out of focus and blurred.

Blue-Light Photochemical Retinal Hazard

The ACGIH TLV3 to protect the human retina against photoretinitis,7 "the blue-light hazard" is an effective blue-light radiance LB of 100 J/(cm2× sr), for t < 10,000 s, i.e.,

LB = Σ Lλ × Bλ × Δλ £ 100 J/(cm2× sr) effective [7]

and for t > 10,000 s (2.8 hrs.):

LB £ 10 mW/(cm2× sr) [8]

To calculate the maximum direct viewing duration when [8] is not satisfied, this maximum "stare time," t-max, is found by inverting Eqn. [7]:

t-max = 100 J/(cm2× sr) / LB [9]

For very small sources that subtend a viewing angle less than αMIN, which is 11 mrad = 0.011 rad. The blue light hazard is evaluated by mathematically weighting the spectral irradiance, Eλ, against the blue-light hazard function to obtain EB to give:

EB = Σ Eλ × Bλ × Δλ £ 10 mJ/cm2 for t £ 10,000 s [10]

and for t > 10,000 s (2.8 hrs.):

EB £ 1 μW/cm2 [11]

To calculate the maximum direct viewing duration when [11] is not satisfied, this maximum "stare time," t-max, is found by inverting Eqn. [10]:

t-max = 10 mJ/(cm2× sr) / EB [12]

Retinal Photochemical Hazard to the Aphakic Eye.

The third type of retinal hazard--the aphakic photochemical retinal hazard--is evaluated by spectrally weighting the radiance against the aphakic retinal hazard function Aλ.18 This photochemical retinal injury hazard is merely an extension of the blue-light hazard and must be analyzed only for individuals with at least one aphakic eye (i.e., an eye with the normal lens removed, as in cataract surgery). The approach is to substitute Aλ for Bλ in Eqns. [7] through [12]. For example, the aphakic hazard radiance L-aphake is:

L-aphake = E-aphake/Ω [13]

LA = Σ Lλ × Aλ× Δλ £ 100 J/(cm2× sr) effective for t £ 10,000 s (2.8 hrs.). [14]

Infrared Radiation Hazards to the Eye

Any calculation of potential retinal thermal hazards to the eye normally includes a consideration of the contributions of IR-A (700-1400 nm) and IR-B (1.4 μm-3.0 μm). In contrast to blue light, IR-A is very ineffective in producing retinal injuries (Ham, et al., 1982, 1976).1,3 The data which could be used as the basis of an exposure limit for chronic exposure of the anterior of the eye to infrared radiation are very limited. Sliney and Freasier (1973) stated that the average corneal exposure from infrared radiation in sunlight was of the order of 1 mW/cm2.8 Glass and steel workers exposed to infrared irradiances of the order of 80-400 mW/cm2 daily for 10-15 years have reportedly developed lenticular opacities.8

The ACGIH guideline for IR-A exposure of the anterior of the eye is a time-weighted total irradiance of 10 mW/cm2 for exposure durations exceeding 1,000 s (16.7 minutes). Pitts, et al. (1979) showed that the threshold radiant exposures to cause lenticular changes from IR-A were of the order of 5000 J/cm2.7 Threshold damage irradiances were at least 4 W/cm2. There is also a second ACGIH criteria to protect the retina against thermal injury from viewing specialized infrared illuminators which have visible light filtered out so that the aversion response stimulus is not present.1

RADIOMETRIC MEASUREMENTS REQUIRED

To evaluate the potential optical radiation hazard to the eye, the ultraviolet spectral irradiance at from 200 - 400 nm would be determined at the nearest location of the eye. Spectral irradiance and radiance of the light emitted from the source in the 400 - 770 nm range (and sometimes to 1,400 nm) may also be required to analyze potential retinal hazards to an observer. Spectral irradiance at longer wavelengths could also be measured, although a measurement of total irradiance in this region is sufficient. The spectral radiance can be determined by measuring the spectral irradiance at a fixed distance (e.g., 30 cm) and dividing by the solid angle Ω subtended by the source.

LB = EB/Ω [15]

The spectral radiance is then independent of viewing distance because of the law of conservation of radiance.

Whenever spectroradiometric measurements are made for the purpose of a safety study, it is imperative that errors are not introduced. For this reason, it is useful to check measured spectroradiometric values with check-measurements made with illuminance and spot-luminance measurements. This is cone by also calculating the illuminance Ev or luminance Lv from the spectral irradiance measurements, e.g.,

Ev = 683 Σ Vλ × Eλ× Δλ [16]

The luminance Lv is then the illuminance divided by the angular subtense of the source Ω:

Lv = Ev/Ω [17]

where the luminance would be expressed in cd/cm2 if the illuminance was expressed in lm/cm2.

 

REFERENCES

American Conference of Governmental Industrial Hygienists 1993 TLV's. 1993. Threshold Limit Values and Biological Exposure Indices for 1993-1994. American Conference of Governmental Industrial Hygienists. Cincinnati, OH.

ACGIH. 1991. Documentation for the Threshold Limit Values, 4th Edn. American Conference of Governmental Industrial Hygienists. Cincinnati, OH.

Ham, W. T., Jr. 1989. The photopathology and nature of the blue-light and near-UV retinal lesion produced by lasers and other optical sources. In: M. L. Wolbarsht, (ed.). Laser Applications in Medicine and Biology. Plenum Publishing Corp. New York.

Ham, W.T., Jr., H.S. Mueller, and D.H. Sliney. 1976. Retinal sensitivity to damage by short-wavelength light. Nature. 260(5547): 153-155.

IRPA. International Non-Ionizing Radiation Committee. 1991. Guidelines for Limits of Human Exposure to Non-Ionizing Radiation. MacMillan. New York.

Pitts D.G. and A.P. Cullen. 1981. Determination of infrared radiation levels for acute ocular cataractogenesis, Albrecht von Graefes Arch Klin Ophthalmol. 217:285-297.

Sliney D.H. and B.C. Freasier. 1973. The evaluation of optical radiation hazards. Applied Opt. 12(1):1-24.

Sliney D. H. and M. L. Wolbarsht. 1980. Safety with Lasers and Other Optical Sources. Plenum Publishing Corp. New York.

World Health Organization (WHO). 1982. Environmental Health Criteria No. 23. Lasers and Optical Radiation, joint publication of the United Nations Environmental Program, the International Radiation Protection Association and the World Health Organization, Geneva.

 

 

Sliney, D.H. 1994. Ocular hazards of light, p 183-189. In: T.W.Tibbitts (ed.). International Lighting in Controlled Environments Workshop, NASA-CP-95-3309. 

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