A filter (10) for selectively transmitting electromagnetic energy over a range of frequencies (28, 30, 32) adapted for use with
white light (24). The filter (10) includes rugate
layers (18, 20) for creating a
resonant cavity that resonates at desired bandpass frequencies (28, 30, 32). An absorptive layer (12) absorbs frequencies near the bandpass frequencies (28, 30, 32) and reflects frequencies outside the bandpass frequencies (28, 30, 32).
Phase matching layers (14, 16) allow the transmission of electromagnetic energy within the transmission bands (28, 30, 32) through the
absorption layer 12. In an illustrative embodiment, the bandpass frequencies (28, 30, 32) comprise the three tristimulus frequencies, i.e., red (28), green (30) and blue (32) frequencies. The rugate
layers (18, 20) include first (18) and second (20) rugate layers made of SiO2 and Ta2O5, respectively. Located between the first (18) and second (20) rugate layers is the
absorption layer (12) that is surrounded by the first (14) and second (16)
phase matching layers. The
absorption layer (12) is a silver layer that is approximately 500 angstroms thick. The first rugate layer (18) has a first index of
refraction versus
layer thickness profile representing a superposition of sinusoids. Each sinusoid has a frequency directly corresponding to tristimulus band frequency. The second rugate layer (20) has a second index of
refraction versus
layer thickness profile that is a phase-adjusted version of the first index of
refraction versus
layer thickness profile. Both rugate layers (18, 20) have index or refraction versus layer thickness profiles have several cycles.