Manipulating Light

Diffusion

Fig. 4.1 Diffuse transmission and reflectance.

It is often necessary to diffuse light, either through transmission or reflection. Diffuse transmission can be accomplished by transmitting light through roughened quartz, flashed opal, or polytetrafluoroethylene (PTFE, Teflon). Diffusion can vary with wavelength. Teflon is a poor IR diffuser, but makes an excellent visible / UV diffuser. Quartz is required for UV diffusion.

Integrating spheres are coated with BaSO4 or PTFE, which offer >97% reflectance over a broad spectral range with near perfect diffusion. These coatings are, however, quite expensive and fragile.

Collimation

Some lamps use collimating lenses or reflectors to redirect light into a beam of parallel rays. If the lamp filament is placed at the focal point of the lens, all rays entering the lens will become parallel. Similarly, a lamp placed in the focal point of a spherical or parabolic mirror will project a parallel beam. Lenses and reflectors can drastically distort inverse square law approximations, so should be avoided where precision distance calculations are required.

Transmission Losses

When light passes between two materials of different refractive indices, a predictable amount of reflection losses can be expected. Fresnel’s law quantifies this loss. If nl = 1.5 between air and glass, then rl = 4% for each surface. Two filters separated by air transmit 8% less than two connected by optical cement (or even water).

Precision optical systems use first surface mirrors to avoid reflection losses from entering and exiting a glass substrate layer.

Focusing Lenses

Lenses are often employed to redirect light or concentrate optical power. The lens equation defines the image distance q, projected from a point that is a distance p from the lens, based on the focal distance, f, of the lens. The focal distance is dependent on the curvature and refractive index of the lens. Simply put, all rays parallel to the optical axis pass through the focal point. Since index of refraction is dependent on wavelength, chromatic aberrations can occur in simple lenses.

Mirrors

Fig. 4.5 Reflection from a second surface and first surface mirror.

When light reflects off of a rear surface mirror, the light first passes through the glass substrate, resulting in reflection losses, secondary reflections, and a change in apparent distance. First surface mirrors avoid this by aluminizing the front, and coating it with a thin protective SiO coating to prevent oxidation and scratching.

Concave Mirrors

Fig. 4.6 Reflection from a spherical concave mirror.

Concave mirrors are often used to focus light in place of a lens. Just as with a lens, a concave mirror has a principal focus, f, through which all rays parallel to the optical axis pass through. The focal length of a spherical concave mirror is one half the radius of the spherical surface. Reflective systems avoid the chromatic aberrations that can result from the use of lenses.

Internal Transmittance

Filter manufacturers usually provide data for a glass of nominal thickness. Using Bouger’s law, you can calculate the transmission at other thicknesses. Manufacturers usually specify Pd, so you can calculate the external transmittance from internal transmittance data.

Prisms

$Fig. 4.8 Refraction through a prism.$ Prisms use glass with a high index of refraction to exploit the variation of refraction with wavelength. Blue light refracts more than red, providing a spectrum that can be isolated using a narrow slit.

Internal prisms can be used to simply reflect light. Since total internal reflection is dependent on a difference in refractive index between materials, any dirt on the outer surface will reduce the reflective properties, a property that is exploited in finger print readers.

Diffraction Gratings

$Fig. 4.9 Diffraction grating.$ Most monochromators use gratings to disperse light into the spectrum. Gratings rely on interference between wavefronts caused by microscopically ruled diffraction lines on a mirrored surface. The wavelength of reflected light varies with angle, as defined by the grating equation, where m is the order of the spectrum (an integer).