[Laser] filter response

[J. Forster]

TWOSIG at aol.com wrote:

> I cannot tell you how they work, but I saw the pictures my late wife took in
> the 60's or 70's to test a haze filter for her 35mm camera.  We were standing
> on the beach on Padre Island on a partly cloudy day.  There was a lot of haze
> looking out over the Gulf.  I looked through the filter and could not see any
> difference in the amount of haze.  When the pictures came back the ones with
> the filter showed almost no haze, but the ones without showed the haze that I
> remembered.  I am pretty sure the film was for color prints.   I think she did
> a similar test on Black and White film.  I remember the dramatic difference
> between what I saw and what the camera recorded.

Haze filters block UV which fogs the film's yellow layer with color film. Google
yields:

http://www.geog.ucsb.edu/~jeff/115a/lectures/films_and_filters.html

or

http://www.photo.net/equipment/filters/

> The description of the filter for a gun camera suggests to me that it was
> used by Navy aircraft at low altitudes.

UV increases with altitude. Gun cameras were also used in dogfights, hence the
requirement for filters. In fact, red would give better contrast of a plane against a
blue sky, making it black.

>  I do not think that haze is as much of a
> problem in thin dry air.

UV increases with lower humidity. Yor burn much more quickly in high desert.

-John


> You might try measuring the absorbtion of light from some red, yellow, green,
> blue (maybe white and ultra-violet) LEDs with and without the filters.  I
> would guess that the photo sensor should be fairly broad - maybe a CdS
> photoresistor.
>
> Have fun.
>
> James
> N5GUI