A calibrated gray scale for forensic ultraviolet photography.

The human eye is an important tool for observing evidence, and visual evidence in turn may be documented onto a photographic medium. The human eye is generally sensitive only to a narrow band of the electromagnetic spectrum from about 700 nm (red) to about 400 nm (violet/blue). It is possible to extend the range of radiations over which evidence may be documented by using the natural ultraviolet sensitivity of photographic films. However, photographing evidence with ultraviolet radiation ultimately involves presenting the information to subjects who may have no prior experience at viewing these wavelengths. This study shows that it is necessary to use a calibrated ultraviolet reflecting gray scale to allow meaningful interpretation of results.

Aquatic vision and the modulation transfer properties of unlighted and diffusely lighted natural waters.

The modulation transfer function (MTF) of well-mixed unlighted and diffusely lighted samples of clear natural waters for path lengths up to 4 m was measured. The measuring conditions simulated the situation for horizontal aquatic vision. In unlighted water, the MTF decreased relatively slowly with increasing path length and spatial frequency up to 150 c/deg. We interpret this as the result of low-angle forward scattering of light from the target. For diffusely lighted water, the MTF fell much more rapidly with path length, but was nearly independent of spatial frequency. Here, scattering of ambient light into the light path contributes an additional veiling glare or path radiance, which is independent of spatial frequency but degrades the MTF strongly with increasing path length. These scattering processes are independent of wavelength in the visible spectrum. The modulation transfer properties of these waters do not preclude high aquatic visual acuity. However, the useful range of high acuity vision in diffusely lighted water is much less than for unlighted water. This places special requirements upon the design of high-acuity aquatic eyes.

Microspectrophotometric determinations of rod visual pigments in some adult and larval Australian amphibians.

Visual pigments from the red rods of adults of eight species of Australian anuran amphibians, from a variety of habitats, were analyzed by microspectrophotometry. The lambda max in all cases fell between 502 nm and 506 nm, and the absorption spectra were well fitted by an A1-based visual pigment template curve. Red rod pigments were also analyzed for a number of tadpoles. In some cases the data were best fitted with an A1-based visual pigment template, in other cases with an A2-based template, and finally some tadpoles appeared to have mixtures of the two pigments.

Annual variations in the visual pigments of brown trout inhibiting lochs providing different light environments.

Visual pigments were extracted at regular intervals over the year from trout (Salmo trutta) inhabiting three Scottish lochs. Measurements of the spectral quality of the light in the lochs were also made. In all cases only A2-based pigment was found in the winter, with A1-based pigment appearing as well in summer. Fish from Loch Turret had significantly less A1-based pigment than fish from the other two lochs. Loch Turret differs from the other two lochs in being dystrophic, as opposed to eutrophic, and the light penetrating into it has more long wavelength energy. Possible correlations between this environmental difference and the visual pigments of the three trout populations are discussed.

Visual adaptations to different light environments in Amazonian fishes.

Light measurements were made at seven sites in the Amazon river system near Manaus. It was found that very little light penetrates into the white water of the Solimões because of the large amount of suspended material it contains. In lakes off the Solimões the suspended material settles out, and light penetration is increased. The black waters of the Rio Negro contain dissolved material but little suspended matter, and the penetrating light is mainly of long wavelength. Fishes were also collected from these areas, and the spectral characteristics of their visual pigments, lenses and corneas measured. The visual pigments of the different species vary considerably, fishes from clearer water having pigments absorbing at shorter wavelengths on average than those inhabiting more coloured or turbid waters, and the characins having pigments absorbing at longer wavelengths on average than the cichlids. Yellow lenses and corneas are common among cichlids, but rare in other groups and fishes with such filters were, with one exception, absent from the Solimões where the light levels are especially low. The exception was Colomesus asellus which has an occlusable yellow cornea, which while coloured in the light loses its colour at low levels of illumination. Using the date on light penetration and visual pigments it was calculated that vision would be impossible below 2.3 m in the Solimões, but should be possible at depths up to about 9 m in the clearest waters studied.