Curvature measurement with reflected-light microscopy and its application to fly facet lenses

Two methods for microscopically measuring the curvature of strongly curved surfaces are compared: one using a Michelson interferometer-type microscope and one using a novel reflectometrical method implemented in an epi-illumination microscope. The curvature values obtained with the two methods were very similar, but the latter proved to be by far the simplest. Curvature measurements on the front surface of the facet lenses of various dipteran flies revealed that facet lens diameter and radius of curvature are linearly related over a wide range of facet lens sizes.

Light-dependent pigment migration in blowfly photoreceptors studied by in vivo CLSM

The light-dependent migration of pigment granules in the soma of fly photoreceptors has been studied in vivo with a fast confocal laser scanning microscope. Images as well as photometric measurements were obtained in the reflection and fluorescence modes. Measurements at the single cell level were performed by using water immersion. The illumination of dark adapted photoreceptors causes a rapid increase in reflectance due to the migration of light scattering pigment granules toward the rhabdomeres. In the steady-state, the reflection signal strongly fluctuates, indicating that the pigment granules undergo a very rapid fluctuating movements. A major part of the reflection signal is due to light back-scattered by the pigment granules and channeled through the light guiding rhabdomeres. The optical axes of the rhabdomeres can thus be directly traced and appear to be directed toward the optical centre of the corresponding facet lens. Simultaneous with the reflection increase, the fluorescence of the photoreceptors decreases, because the pigment granules accumulating near the rhabdomeres act as a light-controlling pupil. Broad-band, white light filtered by the predominantly blue absorbing pupil causes an increased fraction of the visual pigment in the rhodopsin state.

Colors and pterin pigmentation of pierid butterfly wings

The reflectance of pierid butterfly wings is principally determined by the incoherent scattering of incident light and the absorption by pterin pigments in the scale structures. Coherent scattering causing iridescence is frequently encountered in the dorsal wings or wing tips of male pierids. We investigated the effect of the pterins on wing reflectance by local extraction of the pigments with aqueous ammonia and simultaneous spectrophotometric measurements. The ultraviolet-absorbing leucopterin was extracted prominently from the white Pieris species, and the violet-absorbing xanthopterin and blue-absorbing erythropterin were mainly derived from the yellow- and orange-colored Coliadinae, but they were also extracted from the dorsal wing tips of many male Pierinae. Absorption spectra deduced from wing reflectance spectra distinctly diverge from the absorption spectra of the extracted pigments, which indicate that when embedded in wing scales the pterins differ from those in solution. The evolution of pierid wing coloration is discussed.

Dioptrics of the facet lenses of male blowflies Calliphora and Chrysomyia

1. The dioptrics of the facet lenses of two blowfly species, Calliphora erythrocephala and Chrysomyia megacephala, was investigated. Measurements were performed on facet lenses ranging in diameter from 20 to 80 µm. 2. The radius of curvature of the front surface of the facet lenses, measured by microreflectometry, increases approximately linearly with the facet lens diameter. 3. The optical path difference of the facet lens and water, measured by interference microscopy, depends on the distance from the optical axis according to a parabolic function. Average refractive index values, calculated from the optical path difference profile together with estimates of the thickness profile, are between 1.40 and 1.43, with the lowest values in the largest lenses. 4. The F-number calculated from the experimental data ranges from 1.5 to 2.2. It is argued that the range of effective F-numbers is 2.1-2.4.

Dioptrics of the facet lenses in the dorsal rim area of the cricket Gryllus bimaculatus

1. The optics of the corneal facet lenses from the dorsal rim area (DRA) and from the dorso-lateral areas (DA) of the compound eye of the cricket Gryllus bimaculatus were studied. 2. The DRA of the cricket eye contains quite normally shaped facet lenses. The diameter of the facet lens in the DA is 2-fold larger compared to that in the DRA. The radius of curvature of the front surface is distinctly less in the DA facet lenses, as the surface of the facet lenses in the DRA are virtually flat. 3. The averaged axial refractive index of the facet lenses of Gryllus bimaculatus, measured by interference microscopy, was 1.496 ± 0.008 (n = 42) in the DRA and 1.469 ± 0.004 (n = 39) in the DA. The geometrical thickness of the lenses was calculated to be 77 ± 3 µm (n = 42) in the DRA and 56 ± 1 µm (n = 39) in the DA. 4. Analysis of the diffraction pattern obtained with a point light source revealed distinct focusing properties of both the DRA and the DA facet lenses; striking Airy-like diffraction patterns were obtained in both cases. 5. Focal distances measured directly at the backfocal plane were 40 ± 8 µm (n = 84) in the DRA of all the animals studied, and 60-90 µm (n = 62) in DA depending on the animal. Analysis of the diffraction of the point light source yielded very similar focal distances: 40 ± 5 µm (n = 10) in DRA and 81 ± 8 µm (n = 11) in DA. In the DRA, focal distance of the facet lenses was smaller than the cone length, 58 ± 3 µm (n = 9) while in the DA the focal distance matched the effective cone length, 71 ± 5 µm (n = 16).

Imaging scatterometry of butterfly wing scales

We describe an imaging scatterometer allowing hemispherical reflectance measurements as a function of the angle of incidence. The heart of the scatterometer is an ellipsoidal reflector, which compresses the hemispherical reflection into a cone-shaped beam that can be imaged by a normal optical system. The instrument’s performance is illustrated by measurements of the scattering profiles of the blue-iridescent dorsal wing scales of the nymphalid Morpho aega and the matte-green ventral wing scales of the lycaenid Callophrys rubi.