+************************************************************************+
Luminous Eyes. In 1889, Mr Friese Green made the startling discovery that
it was possible to obtain an image upon a sensitive plate by emanations
from the human eye. Mr Greene had an electric arc of 2,000 candle-power,
at which he gazed with one eye for the space of fifteen seconds. Switching
off the light he took a very quick plate and held it close to the exposed
eye for a minute or more; on developing the plate, a spot came up which,
when put under a microscope, showed a distinct image of the arc. Mr Greene
attempted no explanation of this phenomenon, wisely leaving it until other
experiments were forthcoming. He, however, strongly advised no repetition
of the experiment with a 2,000 candle-power arc, for he had suffered from
a black spot hovering about his retina for days afterward, this fact
indicating that the retina is the source of the phenomenon. The matter
dropped, mainly owing to the failure of others to obtain a a similar
effect, the general opinion being that Mr Friese Greene possessed an eye
of particular composition.
**************************************************************************
On a more serious note, ultraviolet light is wonderful thing that keeps us
all alive, but too much of the wrong sort can be dangerous.
As everybody should remember from high school physics, the electromagnetic
spectrum consists of
COSMIC GAMMA X-RAYS ULTRA- VISIBLE INFRARED RADIO
RAYS RAYS VIOLET Blue<>Red WAVES
| |
/ \
Ionising / \ Heat
Radiation / \ Radiation
/ \
/ \
/ \
/ \
/ \
| FAR | UVC | UVB | UVA |Violet
| UV | Short wave | Middle | Near or long |
| | | "Blacklight" |
5 100 200 280 320 400
| Wavelength (nanometers)
Vacuum |only reaches| more active spectrum - |
required for | surface - | penetrates and is |reachs
transmission | cornea | and is absorbed by lens |retina
or conjunctiva
Eye | Corneal injury | As left | As left | Retinal
hazards | conjunctivitis |+ cataracts| with excessive| injury
| photokeratitis | | exposure | (extreme
Active spectrum exposure)
250<---^---------->320
270 (peak) | |
Safety standards: | 3.0 mJ/sq cm | 1.0J/sq cm for|
| over active | 17 min maximum|
| envelope | exposure |
(Adapted from Frank Wilkinson, "Medical Aspects of Ultraviolet Radiation,"
Australian Science, Mar. 1986; and UV-A: Biological Effects of Ultraviolet
Radiation with Emphasis on Human Response to Longwave Ultraviolet, John A.
Parrish et. al., New York: Plenum Press, 1978).
UVA used to be considered relatively innocuous compared to UVB (UVC
"germicidal" souces are asking for trouble, and fall below the absorption
spectrum of both iron and chromium anyway), but UVA is still capable of
producing both erythema (sunburn) and photokeratitis (inflammation of the
cornea) even if the amount of energy required to produce an effect is much
greater. Some high-intensity artificial sources of UVA can exceed the
output of the sun over the 320-400 nm band:
SOURCE mW per sq. cm
Tropical sunlight at noon 5.0-6.0
Sunlight at 3 PM or 9 AM 2.5-3.5
500 watt photoflood at 2.5 cm. 1.0-2
Fluorescent blacklight (F40BL) at 2.5 cm 5.0
Special medical 40W 2 ft fluorescent tube
with internal half silvered reflector 18
1000W Xe-Hg compact arc at 10 cm 50-80
Since the eye is incapable of sensing ultraviolet radiation, the aversion
response of the eye to sunlight may not occur with these sources. Dark
glasses may simply make things worse as they cause the pupil to dilate,
thereby allowing greater amounts of UVA to enter the eye.
It's also important to note that the light output of a given source can
range over a wider spectrum than you expect. For example, with long
exposures using cheap 20W blacklight fluorescent tubes I'm sure I can
smell ozone, which is only supposed to be generated by ultraviolet light
from atmospheric oxygen at wavelengths below 250 nm. Now the light from a
fluoro tube is supposed to be formed mainly from the mercury discharge
emission at 253.7 nm, which the ultraviolet emitting phosphors coated
inside the tube absorb and re-emitt as longer wavelength light. Ordinary
glass made from soda lime tends to absorb some UVA light and has a
transmission cutoff point near the 320 nm boundary between UVA and UVB -
special quartz glass has to be used for lower wavelengths. Yet some UVC
must be getting through.