Chapter 1, The Reptilian Spectacle - UWSpace - University of ...

% Transmittance
#!!"
+!"
*!"
)!"
(!"
'!"
&!"
%!"
$!"
#!"
,-./.0"#(!123"#"4566/"
,-./.0"#(!123"$"4566/4"
78/095.-:6";?90@AB"C?>:3"#"4566/"
78/095.-:6";?90@AB"C?>:3"$"4566/4"
!"
!" $!!" &!!" (!!" *!!" #!!!" #$!!" #&!!"
Wavelength (nm)
Figure 2-2. Transmission spectra of photographic films used as NIR filters. Single sheets
provide insufficient blockage of the visible spectrum: Ektachrome duplicating film because it passes far
red wavelengths, and Portra 160VC because it has several windows of transmission in the visible
range. Double layering either film provides sufficient blockage of the visible spectrum, though this also
attenuates some NIR, necessitating an increase in the gain of the imaging system.
Using photographic film as a NIR filter has the advantage of cost and adaptability of the
material - it is thin, deformable and easy to cut to shape. The drawback is that in addition to blocking
the visible spectrum, it also attenuates some NIR, effectively acting as a neutral density filter that
limits the brightness of the image obtained.
Because non-visible illumination was used, the camcorder was modified by replacing its NIR-
blocking filter with a clear glass window of equal thickness. Using a custom-fashioned filter adapter,
the camera was mounted by its filter thread to a beam splitter on the slit-lamp which permitted
monitoring of the image on its LCD screen. With the camera’s zoom lens set to the longest focal length
and using the 40x objective of the slit lamp, the system was able to resolve a lower limit of
approximately 12 µm which is just sufficient to resolve individual erythrocytes that measure ~12.6 µm
on the short axis in M. flagellum (Hartman & Lessler 1964).
37