a trichromatic colorimeter with spectral primaries - InfoCom

W. D. Wv&ht
242
As a general colorimeter for use in industrial laboratories an instrument of this type
has, however, many disadvantages. Many of the adjustments and calibrations
which would have to be made from time to time with such an instrument are only
possible in a research laboratory, requiring auxiliary equipment for the standardisation
of wave-length, photometric transmission, etc., which would not be available
in an industrial laboratory. Colorimeters with coloured filters as primaries have the
advantage that, when once calibrated at a standardising institution, nothing which
can affect the essential constants of the apparatus can possibly go wrong for a very
long time.
Mr R. Kingslake (communicated) : Mr Wright is to be very heartily congratu-
lated on this extraordinarily ingenious colorimeter, and particularly on the many neat
devices he has thought of to overcome the various difficulties which he encountered.
I had the privilege of watching the instrument grow in its early stages, but many of
the parts are new. Especial mention should be made of his idea of using small
reflecting prisms instead of bits of flat mirror, to remove the streakiness of the field
and to make accurate rotational adjustment of the little reflectors unnecessary.
I should however like to ask what is the width of these prisms, or rather what length
of spectrum is reflected by them? Surely these bands are too broad to be considered
as providing strictly monochromatic primaries ?
The author has very well emphasised the importance of removing stray light,
and some of his methods are extremely ingenious, displaying a very intimate know-
ledge of the underlying action of the whole instrument. Another point which he has
considered very carefully is experimental comfort ; this is of course important in any
research, but especially so in photometric or other critical visual work. I may add
that Mr Wright not only designed all his apparatus but also made much of it him-
self in the College workshop.
In conclusion I must express admiration at the clearness and completeness of
the whole description, with the hope that Mr Wright will shortly be able to contri-
bute further papers on the various applications to which he can put the apparatus.
Mr Wright: Mr Guild rightly points out that the gelatine wedges need frequent
re-calibration and whilst this is inconvenient it is a difficulty which has to be over-
come at the expense of a certain amount of time. When permanent local irregu-
larities develop, the wedges will have to be replaced by new ones. The use of black
glass wedges would be an improvement and would be a necessity for instruments
of a more permanent nature than the present one.
With regard to the uncertainty introduced by the use, as the test colour, of a
beam which is not strictly monochromatic, the magnitude of the error will need
investigation and, if greater than can be tolerated, it will either be necessary to de-
termine a correction curve, or eliminate the error by some instrumental modification.
In reply to Mr Kingslake’s query, the size of the prism in the spectrum does
not determine the range of wave-length that is included in the beam entering the
eye; the determining factor is the size of exit-pupil, as is pointed out in the course of
the paper.