Although used in every theatre, conventional plastic color filters have many disadvantages: the filtering of wavelengths characterizing the desired color is not very precise, and a noticeable amount of light is wasted in transmitting those wavelengths. The filters are not very tolerant to heat, and they tend to become charged with static electricity, which causes them to accumulate dust - creating an additional barrier to light transmission. Dichroic filters, which rely on state-of-the-art thin film technology solve these problems to a large extent. Nevertheless they are extremely expensive, a factor which limits their use.
A dichroic filter consists of a very thin film of crystals sandwiched between two sheets of Pyrex glass. The desired wavelengths are transmitted with great precision and the efficiency of
transmission
is high so that there is little loss of light intensity. The filters are also heat resistant and do not fade with extended use so that, barring breakage of the glass, they will last indefinitely.
A dichroic filter is a semi-transparent mirror which enables
selective transmission
of light while reflecting (as opposed to the absorption of conventional filters) the undesired wavelengths of light, as well as other electromagnetic
radiation
.
Another optical characteristic of dichroic filters is that they produce an optical shift of color, which occurs when rays of light hitting the filter are not perpendicular to the surface of the filter.
Due to their high price dichroic filters are used in theatre principally in automated lights, which require filters of small diameters located on a wheel or other mechanism inside the body of the luminaire . Recently, dichroic filters have been produced in various square shapes suitable for use with PAR luminaires. At present the range of colors is still limited, but as the lighting equipment industry grows and the demand rises, prices should drop, and the new technology will undoubtedly come into wider use.
Dichroic filters are also used as reflectors for special lamps such as MR or Par 64 discharge lamps. The
reflector
which lies behind the
lamp
is in fact a dichroic filter which transmits the
infrared
radiation away from the
lamp
while reflecting the visible light forward. This produces a cooler
beam
of light while protecting the
lamp
from excessive heat. Dichroic reflectors, also known as cold mirrors, also solve the problem of reflectors deteriorating due to heat.
Dichroic filters are very useful in architectural lighting and in permanent installations where the luminaires are almost inaccessible, so that the durability and color consistency of the filter become primary considerations.
