Solar extinction

measurement system

based on digital cameras

A novel measurement method based on

digital images and Lambertian target

Atmospheric extinction of solar radiation reflected by heliostats to receiver is recognized as an important

cause of energy loss in the increasingly large solar tower plants. During the design of these plants, it

would be desirable extinction maps similar to the solar resource ones. Unfortunately, the truth is that

there is currently no reliable measurement method for solar extinction and, at present, these plants are

designed, built and operated without knowing this local parameter. The aim of this measurement system

developed by the Almeria Solar Platform (PSA), is to take simultaneous images of the same target at very

different distances using two identical optical systems with digital cameras, suitable lenses and filters.

Having Lambertian targets is important for this measurement system since the position of the cameras

and the angles relative to the target can never be known for sure at such large distances.

System configuration and set-up

The distances of the cameras to the center of the target are

chosen so that the target area projected per pixel is similar

in both. Hamamatsu™ cameras with high resolution and

accuracy are used.

Positioning the cameras at a very large distance between

them and taking images of the target, the intensity levels of

the digital images would be proportional to the diffuse

radiance coming from the target. The difference of intensity

of these images will be due to the extinction of the radiation

in the path between the two cameras. The system can

measure solar extinction in the bandwidth 400 -1000 nm

with an accuracy better than an absolute ±2%.

Image courtesy of PSA

Solar extinction measurement system

� A dedicated software selects same areas of the target from the two cameras and luminous intensity

is measured.

� The intensity difference or light extinction between both images, provides the extinction for the

experimental distance, being βExt the extinction coefficient considering both the absorption and

scattering mechanisms, and dependent on humidity, aerosols and particle concentration.

EN-BCB358-3-1811

Why a Lambertian target?

For solar extinction measurements systems, the positions of the cameras and angles relative to the

target can never be exactly known at such large distances. With a perfect diffuser surface that reflects

solar radiation isotropically, an image of the target taken from any viewing angle will accurate represent

the same brightness or luminance. A Lambertian surface reflects or emits equal luminance in every

direction. The fact that the target is a perfect diffuser allows neglecting the directional influence of the

reflected radiation with respect to the position of the camera.

Polanco V,

11560 Ciudad de México

bcb México