A transmissometer, also referred to as a telephotometer, hazemeter, or transmittance meter, is a device that is used in determining runway visual range (RVR). The transmissometer measures the extinction coefficient (the absorption of light through a medium) by using a laser shot through the atmosphere to other transmissometers at a predetermined distance. The detector determines how much energy is arriving from the laser beam, and from this, it figures out the extinction coefficient. The transmissometer usually broadcasts a wavelength of 550 nm, which is in the middle of the visual waveband spectrum. This allows for more than a decent estimate of the RVR. They are installed on one side of the runway and normally on the ends and in the middle to provide accurate runway distance information.
A transmissometer works off the principal of transmittance. This is the fraction of light that is present after passing through a medium. For example, if you shine a flashlight through a piece of dark construction paper, light still shines through, but it is not as bright. The transmissometer measures how much light still makes it through the atmosphere and determines how much visibility there is on the runway. The worse the atmospheric conditions are, the less light will shine through and the lower the runway visibility will be.
Lasers have helped make many advancements in aviation. They are used for detecting clear air turbulence and wind shear. There are experiments using lasers to detect even microbursts. Since light travels fast, lasers have the ability to go for a seemingly infinite distance without converging. This allows for the exact distance between points to be measured from extremely far away. Light also has velocity, so it can detect anything moving. Lasers have the ability to do both. If air rushes through the laser, a velocity can be determined. Though a transmissometer determines the strength and not velocity or distance, it wouldn’t be a useful tool in aviation without the help of lasers.