It uses a programme
To help working with the table, an overlay is computed as well, showing
the tiles of the photometric table and some of its values. Namely, medians
of the upper green pixels within the Bayer grid of the chip
(or of any pixels at this place) and, if the
exposure is known, the median luminances (or average luminances
or brightnesses of the tiles) as well.
Early 2004, a supplementary
output has been developed, coding the
luminances by a colour scale. The scale spans a range of 1:1E7, going
from 0.318E-3 nt to 3.177E4 nt. In each colour, five luminance steps are
used, with borders at 0.318, 0.504, 0.798, 1.264,
2.005, 3.177. The sequence of colours is blue: milinits,
magenta: centinits, cyan: decinits, red: nits, green: dekanits, yellow:
hektonits, blue again: kilonits, grey: tens of kilonits. Let's remember
that nit is an alternative name for candela per square metre. The
luminances corresponding to the centres of the faint intervals are (on
example of red colour) 0.4 nt, 0.63 nt, 1.005 nt, 1.59 nt and 2.52 nt.
The calibration of luminances for Canon EOS D60 is still rather
preliminary, based at first just on a couple of defocused images of
sunlit and moonlit snow in February and of Capella in zenith (Sun has
been rather low in the sky and the times for the Moon imags are
doubtful). Then some images of summer sunlit white paper had been made
in 2003 and a better calibration obtained. It may be easily 30 % off for
non-gray scenes lit by sodium light.
Calibration for Fuji S5000 is more reliable, being based on using
luxmetres as well. For sunlight and scenes which are not very coloured,
its uncertainty perhaps is about five per cent only (well, for the single
camera I've used).
A more adequate linear combination of R, G and B could be used,
what may be important for sodium lights. Taking a couple of images of
solar spectra in summer might solve this problem, for each camera
individually.
Another source of uncertainty, not over 10 %, are the
exposures (focal ratios or even exposure times which
are not perfectly natching the reported values).
(the images and *.tab contain some results. Pixel values over 3980
seem to mean overflow for D60 -- even if overflow is 4095, some
spillage to another pixels can proceed).
Jenik Hollan, partial update on Jan 2004.
dcraw152j
modified by me from the outstanding
dcraw. Another option is to use unmodified
decompress, but this works just for Canon cameras.
The other input is from
jhead. See
David Coffin's page for sources of standard versions of the programmes.
From their output, a photometric table is computed using
raw2lum.