K3's Astronomy - Exposure
"When I consider your heavens, the work of your fingers, the moon and the stars which You have set in place, what is Man that You are mindful of him?" -- Psalm 8:3,4

As an amateur astronomer I am using webcam for astrophotography. It is Philips Vesta Pro (PCVC680K) model. It uses SONY ICX098AK CCD chip.
Image acquired from Vesta Pro webcam is 24-bit RGB bitmap (AVI file uses 12-bit codec). Although 24-bit BMP format has 16777216 possible colors, it has only 256 levels of brightness - it means, that the ratio between the brightest pixel (level 255) and the detectable darkest pixel (level 1) is only 255:1 (48dB). In magnitude scale it means the difference of 6.01 mag.

The range of brightness levels on Moon' disk during the first quarter is much more than 1:255. More expensive specialised astronomy CCD cameras use 16-bit A/D converters, which guarantee the brightness ratio 65535:1 (96dB). In magnitude scale it means the difference of 12.04 mag. It allows to obtain more precise astronomy photographs, but the problem of displaying such photographs on computer monitor is the same - graphic cards work in 24-bit (eventualy 32-bit) mode, which allows 256 levels of brightness.

The possibility of displaying images with such range of brightness is to convert the original 16-bit levels image to 8-bit levels. Using a simple cutting lower 8 bits will cause similar effect as a picture was captured at 8-bits - the dark parts of image will disappear. Solution of the problem can be converting 16-bit picture to 8-bit according to logaritmic scale - the displayed brightness ratio of result picture will be still 255:1, but darker parts will be visible.

There is a possibility to record more brigthness levels with 8-bit webcam, too. It can be achieved by superposition of two or more pictures of the same object with different exposure settings.

Normal exposure
Over-exposed picture
Superposition of previous 2 pictures (Pic1+50% x Pic2)

The above pictures show the procedure of obtaining picture, which enable to display parts of the moon with higher brightness ratio than 255:1 with webcam. Superposition was done in Corel PhotoPaint
Better results can be achieved by writing the specialised software for combining pictures with different exposure.

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Last Update: 11.4.2001