Damage-weighted retinal irradiance

The intensity of ambient light is commonly measured in ftc. Ftc are a measure of illuminance which is the radiation emission from a light source, weighted for the fact that our eye perceives different wavelengths with different intensities.

For instance, our eye sees yellow brightest, so the weighing assigns a higher value to yellow than the amount of energy radiated in the yellow wavelengths would warrant. If we want to know the true amount of energy radiated, we must eliminate this distorting factor and convert the illuminance into irradiance.

The conversion factor for the spectrum of the nursery lamps can be derived from a survey of lighting levels in eight intensive care nurseries in which workers from the U.S. Food and Drug Administration's Center for Devices and Radiological Health monitored both illuminance and irradiance at the level of the babies (51).

Based on the grand means of these parallel measurements, a "Deluxe Cool White" fluorescent lamp with a baby-level illuminance of 60 ftc bombards the skin and eyes of that baby with an irradiance of 302 micro-Watt per square cm. This is the corneal irradiance.

Corneal irradiance is converted into retinal irradiance with the standard equation for optical image area ratios (52-54):

• Er = Ec times t times de squared divided by f squared,   where:

• Er = retinal irradiance in micro-Watt per square cm

• Ec = corneal irradiance -- 302 micro-Watt per square cm, (see above)

• t = transmission factor for light through the eye: for infants about 90% at 436 nm (here t = 0.9), and 95% for most longer waves (55, 56)

• de = pupil diameter -- about 0.43 cm in a 30-wk gestational age preemie (57, 58), fully dilated because prolonged exposure to bright light redilates the pupil (59, 60).

• f = internal diameter of the retina - about 0.97 cm for the same 30-wk preemie (61).

These values in this formula yield a retinal irradiance of 53.4 micro-Watt per square cm.

This energy must be weighed for the amount of blue-light damage each wavelength- band inflicts on the retina. To weigh the "Deluxe Cool White" light, scale from the spectrum graph of this lamp in Figure 1 the energy levels at the midpoint of every 10 nm bandwidth. They are listed in the fourth column of Table 1 as unweighted irradiance. Multiply each of these numbers with the blue-light hazard value from column 2 of that same Table 1 to obtain the corresponding damage- weighted irradiance which you find in column 5.

The totals show that, for this type of lamp, the damage-weighted irradiance amounts to 20.5% of its unweighted irradiance. The same percentage of the preemie's above-mentioned unweighted retinal irradiance then becomes a damage- weighted retinal irradiance of
11 micro-Watt per square cm.

This damage-weighted retinal irradiance is higher than that which the U.S. occupational exposure limits allow for healthy adult industrial workers. These limits regulate the exposure to laser light but are, as discussed above, just as applicable for exposure to fluorescent light.

The exposure limits are published by the American Conference of Governmental Industrial Hygienists in A Guide for Control of Laser Hazards (62) and by the Laser Institute of America in their Laser Safety Guide (63). Established in 1969, the limits had to be lowered in 1976 and again in 1981 based on NIOSH's action spectrum and in response to growing evidence that light harms the retina at doses much lower than previously suspected.

The current version of these exposure limits does not offer much protection even for relatively robust adults. Instead of including a hundredfold safety factor as in prior exposure limits, the current version allows up to 1/27 of the lowest damage-weighted retinal irradiance which in animal experiments had caused damage after relatively short latency times (42).

People vary greatly in their sensitivity to light, as tanning and sunburn reactions easily demonstrate, and some damage from low-intensity light, particularly in the shorter wavelengths, becomes noticeable only after longer latency times (64) (just like ROP). This safety factor is therefore not necessarily safe enough for all people.

Inadequate as they may be, these occupational exposure limits allow us to compare the mandated light exposure danger limit for adults with the dose of light preemies receive. This dose is a function of the length of exposure because blue-light damage to the retina is a photochemical process (65). The effects of the irradiation accumulate over the duration of the exposure, just as on photographic film.

This accumulation is counteracted by the ability of mature eyes to repair themselves to some decree if the damage is not too great. If the damage from a level of light occurs more slowly than the self-repair mechanism can repair it, no permanent damage remains.

The Laser Safety Guides are, therefore, based on the assumption that photochemical damage accumulates on the retina only damage the first 10,000 seconds of exposure. A level of light which has not overwhelmed the self-repair mechanism in these 167 min is deemed safe (62, 63).

The occupational light exposure calculations multiply the intensity of the irradiation with the length of exposure. The energy hitting the retina is measured in Watt per square cm, so the product accumulates on a given surface in Watt-seconds. One Watt-sec is 1 Joule per square cm.

The Guide for Control of Laser Hazards as well as the Laser Safely Guide give the adult maximum permissible exposure to damage-weighted retinal irradiance for times from 10 to 10,000 sec as 0.01 Joule per square cm.

In 10,000 sec of exposure to the above damage-weighted retinal irradiance of 11 micro-Watt per square cm from 60 ftc nursery lighting, the retina of the preemie absorbs 0.11 Joule per square cm.

The lighting level which the American Academy of Pediatrics currently recommends for intensive care nurseries thus exposes the still developing eyes of the preemies to 11 times the amount of damage-weighted retinal irradiance established by the U.S. Government's occupational safety guidelines as the danger limit for the eyes of healthy adult workers.

Since this 11-fold overdose accumulates within 10,000 sec or less, the preemie's eyes can absorb the adult danger dose in 15 min or less.

Light levels
in some intensive care nurseries

Most preemies can absorb that danger dose in much less than 15 min, because they are often exposed to higher irradiation levels. For instance, the above-mentioned survey measured the nursery illuminance and irradiance levels at a time when the American Academy of Pediatrics recommended 100 ftc.

Some of the nurseries surveyed were even brighter than that, up to an average light level of 138 plus or minus 29 ftc in the unit with the brightest lights. Another report, from a nursery in Seattle, Washington, gave 99 ftc for overcast days and 163 ftc on sunny davs, but with no direct sunlight entering the room (66). These extremely high values were measured in 1980, just before the annual number of babies with ROP at that same Seattle nursery tripled (8).

The surges that are common in most electricity supply grids also can drive up the light levels: when the incoming current changes from, say, 100 to 125 Volt, a fluorescent lamp will more than double its irradiance (51).

An additional source of mostly uncontrolled light are phototherapy units which hang over jaundiced preemies to reduce the concentration of potentially brain-damaging bilirubin in their blood. These units typically combine, in one lightbox, four of the same "Deluxe Cool White" lamps as are on the nursery ceiling and radiate, at the level of the baby, anywhere from 300 ftc, the lowest value given in the reports reviewed (67), to a high of 480 ftc (51).

The U.S. National Research Council told the Committee on Fetus and Newborn of the American Academy of Pediatrics that infants should be protected during phototherapy from eye injuries due to the radiation. The Committee on Fetus and Newborn incorporated this recommendation into its Standards and Recommendations for Hospital Care of Newborn Infants (68).

Unfortunately, the Standards do not take into account the fact that light shines not only down but in all directions, and that there is no light intensity threshold below which photons no longer cause photochemical reactions.

If a bilirubin light endangers the retinae of the baby directly below it, it cannot be safe for the eyes of that baby's neighbor just as few inches away from the light source. The baby directly underneath the bank of lamps usually gets eye patches as protection against the bilirubin lights, but the baby in the adjacent isolette often gets no patches while receiving virtually the same intensity of irradiation.

Direct sunshine can be hazardous to unprotected eyes also. In more primitive times, societies punished some of their worst criminals by making them stare into the sun until their eyes were destroyed. Nowadays some nursery staffs appear unaware of the dangers from sunlight.

A report from a nursery in Washington, D.C., describes how a group of babies near the nursery windows had "on occasion" been left lying with the sun in their faces, exposed to light intensities in excess of 400 ftc. Most of them went blind. The authors of the report computed the chances as 199 in 200 that it was this exposure to sunlight which had blinded the babies (69).

Such carelessness about sunlight is not an isolated case. The above-mentioned nursery in Seattle, for instance, that had the high light levels and a tripling of babies with ROP in the early 1980s, reported measurements of nursery luminance with direct sunlight entering the room. The mean of these measurements taken right next to the isolettes works out to 226 ftc, and the maximum measured was given as 1124 ftc (66).
 

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