A Filter

Spectral Filters:

radiation quantities within specific wavelength intervals, facilitated by filters selectively transmitting defined wavelength ranges.

Common Filters:

U (for ultraviolet), B (for blue), V (for visible, meaning the center of the visible part of the spectrum). More recently, R (for red), and I (for infrared) have been added. (There are actually a couple of filters in different parts of the infrared.)

Insufficiency of Filters:

• Using a filter, is actually measuring the integral of I(λ, T) over some wavelength range. Actually, the situation is more complicated. The transmission of any real filter is not 100% over the selected range, and this must be factored in.

• Continuous spectra of stars do not exactly follow blackbody curves. Therefore, observations through two filters are not generally sufficient to determine the temperature of the star. Over the years, a system of standard filters has been developed, so that astronomers at various observatories can compare their results.

Interference filters: