How about measuring the light transmission of the pinhole with a light
meter? Shine a strong lamp on the pinhole at a known distance between the
lamp and pinhole and pinhole and a screen. Measure the spot on the screen
with a normal lightmeter of adequate sensitivity. Then calibrate by
using a "pinhole" of a calibrated size, say 1 mm. I believe a machinists
#60 drill makes a hole quite near 1 mm. Of course, the distances are kept
the same in both measurements. Don't forget the light intensity goes as
diameter squared, as does the aperture scale of the light meter.
Instead of a screen, you could just put the lightmeter behind the pinhole
(maybe with or maybe without an incident light option). This would offer
more sensitivity but you must be a little careful to correctly hit the
sensor and you should keep in mind that if the sensor is a photoresistor it
may give quite different readings if the same amount of light falls as a
concentrated spot on the sensor as compared to falling fairly well
uniformly over the sensor. The incident light option gets around this and
a photo diode based meter is unlikely to exhibit the problem. A suitable
source can reduce this problem greatly.
Bob
PS How about avoiding the whole issue by making the hole the correct size
by construction. If you push a tapered needle into whatever is receiving
the hole and you know the needle diameter at the largest diameter of the
needle which entered the hole, don't you more or less know the pinhole
diameter. You can measure a needle with a workshop micrometer. You could
also puch the needle into an existing hole until it stops and measure in
the same way. I once made a gauge for small holes working this way,
following a design known for decades if not centuries!
Dr. Robert Mueller
Institut f�r Festk�rperforschung, FZ-Juelich
D-52425 Juelich, Germany
phone: + 49 2461 61 4550 FAX: + 49 2461 61 2610
email: [EMAIL PROTECTED]
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