by Richard Rallison
The effect of high energy radiation on gelatin is also important to anyone intending to put DCG hoes into space or into solar collectors outdoors or into creative lighting designs in buildings where the windows are DCG gratings. Gelatin, like all organic and many inorganic substances will be damaged by photons of more than 4 or 5 electron volts. Eventually all the bonds will be broken if the flux and energy are high enough for long enough. The gel will then be decomposed into loose atoms and molecules incapable of maintaining the original physical form. Fortunately the process takes a very long time to complete, if the gel is protected between two pieces of glass.

We have tested holographic scanners sandwiched between two pieces of 1.5 mm thick glass by placing them near a 250 watt low pressure mercury arc lamp for over 24 hours or until the glass itself solarized and became dark near the exposed surface. The gelatin was unchanged in diffractive properties but the glue line and the gel were slightly yellowed which would reduce transmission in the blue region. Less severe tests on display holograms were carried out from 1977 to 1979 where the test pieces were placed out in the open to take on whatever the weather in Salt Lake city and also in New Jersey could dish out. In all cases, for a year or for a month, the glue that we were using then became noticeably yellow but the gelatin and glass remained unchanged within the limits of our ability to measure it optically.

Another test was carried out on DCG and three other recording materials that were all prepared in our Paradise lab in 1989 for Northrop electronics division. Several Sample gratings were recorded on thick fused silica substrates and then baked and capped with another silica plate. This time the wave-fronts were measured and photographed before and after the radiation treatments. Neither the efficiency or any other property of the hologram changed from high energy radiation equivalent to several years of direct exposure in space. Somewhere a formal report exists to back up this statement but it did not yield to my searches and the individuals that carried out the work are no longer at Northrop. They did send me some photographs of the wave-fronts and I have those but they cannot tell me what the radiation doses were. In this case the silica could not absorb much of the radiation so it was a truer test of the durability of the gelatin itself. A few of our customers have put our large gratings into green houses as panes of glass and they appear to have survived at least since 1986 or so. Several 16 inch diameter hoes have gone to NASA Goddard where they are used in outdoor LIDAR since about 1991. One early reflection hoe which was not baked out has drifted a little to the blue, nothing that was baked thoroughly before sealing has changed.