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10 - 
Spectral Sensitivity of Detector 
Aerial camera lenses are usually designed to work in 
the spectral range 0.5 to 0.7 micron. Some are designed 
for good performance in the near infra-red. The MTF will 
vary with spectral wavelength to an extent determined by 
the design. In general, the highest performance can only 
be obtained by restricting the spectral range and some of 
the best lenses of long focal length are also the most 
sensitive to change of wavelength. 
If the MTF is to be a significant guide to the pract 
ical value of a lens, then clearly it must be determined 
for a spectral energy balance that corresponds to the con 
ditions of use in aerial photography. * While this would 
have to be standardized by agreement, the general prin 
ciple can be defined as follows: - the spectral sensitivity 
of the photocell detector in the MTF measuring apparatus 
must be corrected to match the spectral sensitivity of the 
emulsion with which the lens will be used, and the spectral 
composition of the light in the MTF determination must match 
that of the exposing light in aerial photography. Substant 
ial filtering is necessary to meet these requirements since 
the spectral sensitivity of photomultiplier cells in general 
is quite different from that of panchromatic emulsions. For 
accurate work it will be necessary to have the spectral res 
ponse of the photocell calibrated, since individual cells 
vary appreciably from the nominal response. 
MTF determinations in monochromatic or narrowband ill 
umination, though necessary in design or research, are obv 
iously useless as a guide to the practical performance of a 
lens. Monochromatic determinations in three or more wave 
lengths would give some useful information. 
Specification of Focal Plane 
Just as in measurement of resolving power, the focal 
plane for MTF measurement is specified by reference to some 
optimum plane rather than by any absolute length. For example, 
the reference plane might be given as that focal plane in which 
the on-axis MTF is maximized at 40 cycles per mm. Maximum 
response for a single frequency, well within the passband of 
the lens, is more clearly definable than a maximized total 
MTF, which involves problems of interpretation. The plane 
can be located more accurately by MTF methods than by resol 
ving power. For the time being, this practice is acceptable, 
but it is desirahle to develop methods whereby the plane can 
be referenced by its distance from the lens flange or other 
indexing point. 
* This implies that the emulsion integrates a mixture of 
coloured spread functions in the same way as the detector, 
which has yet to be proved.