Page 162 - The Art and Science of Analog Circuit Design

Tripping the Light Fantastic

ture. Experimenting with a hair dryer, a can of "cold spray" and a pho-
tometer, I found that each lamp seemed to have an optimum operating
temperature range. Excursions above or below this region caused emit-
tance to fall.
I put a lamp into a reassembled display. With the display warmed up in
a 25°C environment I was able to increase light output by slightly venti-
lating the lamp enclosure. This increased steady-state thermal losses,
allowing the lamp to run in its optimum temperature range. I also saw
screen illumination shifts due to the distance between the light entry point
at the display edge and the lamp. There seemed to be some optimum dis-
tance between the lamp and the entry point. Simply coupling the lamp as
closely as possible did not provide the best results. Similarly, the metallic
reflective foil used to concentrate the lamp's output seemed to be sensi-
tive to placement. Additionally, there was clearly a trade-off between
benefits from the foil's optical reflection and its absorption of high volt-
age field energy. Removing the foil decreased input energy for a given
lamp emission level. I could watch input power rise as I slipped the foil
back along the lamp's length. In some cases, with the foil folly replaced, I
could draw sparks from it with my finger!
I also assembled lamps, displays, and inverter boards in various un-
original combinations. In some cases I was able to increase light output,
at lower input power drain, over the original "as shipped" configuration.

Grandpa Would Have Liked it

I tried a lot of similarly simple experiments and slowly developed a
growing suspicion that nobody, at least in my sample of computers, was
making any serious attempt at optimizing (or they did not know how to
optimize) the backlight. It appeared that most people making lamps were
simply filling tubes up with gas and shipping them. Display manufactur-
ers were dropping these lamps into displays and shipping them. Com-
puter vendors bought some "backlight power supply" board, wired it up
to the display, took whatever electrical and optical efficiency they got,
and shipped the computer.
If I allowed this conclusion, several things became clear. Development
of an efficient backlight required an interdisciplinary approach to address
a complex problem. There was worthwhile work to be done. I could con-
tribute to the electronic portion, and perhaps the thermal design, but the
optical engineering was beyond me. It was not, however, beyond Apple's
resources. Apple had some very good optical types. Working together, it
seemed we had a chance to build a better backlight with its attendant
display quality and battery life advantages. Apple would get a more
saleable product and my company would develop a valued customer. And,
because the whole thing was beginning to get interesting, I could get out
of my rut. The business school types would call this "synergistic" or
"win-win." Other people who "do lunch" a lot on company money would

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