The optical measurements of solar radiation inside Titan's atmosphere can reveal a great deal regarding the physical processes occurring there. For example, the absorption of sunlight determines the amount of energy available at different levels in the atmosphere available to drive atmospheric dynamic, e.g., winds.
Now, for an explanation of the instruments..
This instrument measures the light spectrum of upward and downward sunlight as a function of altitude during the descent. The purpose of this data includes measuring the profile of energy absorption, that is, how much sunlight is able to diffuse through Titan's atmosphere and accordingly, how much sunlight reaches the planet's surface. This instrument will also be vital in defining the composition and nature of Titan's surface by measuring the absorption of reflected sunlight. Different surface composition will have diagnostically different spectra. In addition, the visible spectrometer will provide a vertical profile of methane distribution in the atmosphere and near the surface. This instrument will also measure the optical properties, size, and vertical distribution of aerosols (particles in the atmosphere) and clouds.
The IR spectrometer contains two parts: an upward-looking infrared spectrometer (ULIS) and a downward-looking infrared spectrometer (DLIS), which are useful in measuring the upward and downward fluxes. This instrument has much in common with the Visible Spectrometer, but it measures at longer wavelengths than the human eye can see. An internal shutter opens and closes alternately to account for dark current during the descent measurements.
Solar Aureole Camera
This instrument will measure the intensity of scattered sunlight near the sun. The size of this “aureole” of light around the sun is sensitive to particle size. The solar aureole camera will also measure the vertical and horizontal polarization in the sunward and anti-sunward directions as the probe spins. The polarization senses the size of components making up the aerosols. These measurements will be collected at two wavelengths to provide sensitivity over a range of particle sizes and altitudes. In effect, the solar aureole camera will retrieve particle size distribution data for different levels of Titan's atmosphere.
The purpose of the violet photometers is to allow measurements similar to those of the other spectrometers at shorter wavelengths. This extension of measurements enables DISR to cover nearly all the solar energy within Titan’s atmosphere. This instrument is also able to determine the absorption properties of photochemical aerosols in Titan's stratosphere. Like the other spectrometers, the violet photometer contains two parts: and upward-looking and downward-looking portion.
The timing of all the upward-looking DISR measurements are all based on the position of the sun in the sky. The location of the sun provides a standard orientation for the other instruments' data. As the sun crosses this instrument's field of view, it passes through three slits. Each slit produces a “pulse” signal, which appears in the detector. The slits are arranged such that the elevation of the sun in Titan’s sky can be measured. The DISR sun sensor will continue to look for the sun if it disappears behind a passing cloud, so that the other instruments will always have a "baseline" of instrument rotation in Titan’s sky.
Surface Science Lamp
This instrument will illuminate Titan's surface, since very little sunlight is able to reach the surface at many wavelengths. The SSL is a 20-watt lamp designed like a flashlight to produce a beam of light. It is activated at a height of 700 meters above Titan's surface, as determined by the radar altimeter. This instrument will allow continuous measurements of the spectral reflectivity of the surface throughout the spectral range using the downward-looking infrared spectrometer during the last few minutes of the descent.
some diagrams of the DISR instruments