Tunable color enhancement filter
Single-stage polarization interference filters (PIFs) which produce sinusoidal transmission spectra useful for (Red, Green, Blue) color enhancement (CE). The depth of the CE effect can be modulated by rotating a polarizer. Further, a color suppression (CS) (Cyan/Yellow) state is obtained by rotating a polarizer to the orthogonal state, and a spectrally flat neutral state is obtained at an intermediate polarizer orientation. Described are CE filters that are very stable in transmitted lumens and white-point during tuning, which can, for example, substantially eliminate the need for image post-processing. Tunable CE filters can be used for sensors, image capture, display, tunable light sources, augmented reality, virtual reality, and sunglass eyewear applications. Further described are camera filters with a unique two-step tuning mechanism, which allows photographers to make independent adjustments to functional filter characteristics using a single hand in a sequential manner.
1. A variable color enhancement filter, comprising:
an input linear polarizer;
a color enhancing polarization interference filter (PIF) including one or more wavelength dispersive retarders; and
an output linear polarizer positioned so that light passing through the input polarizer and the PIF passes through the output polarizer, wherein the output polarizer is rotatable relative to the input polarizer;
wherein the variable color enhancement filter has a transmission spectrum that varies depending on a rotational position of the output polarizer relative to the input polarizer, and the transmission spectrum generated when the output polarizer is in one rotation orientation includes full color enhancement (CE), with at least one peak in each of the red (600-670 nanometers), green (520-555 nanometers), and blue wavelengths (450-475 nanometers), and at least one minima in each of the cyan (475-520 nanometers) and yellow (555-600 nanometers) wavelengths of maximum modulation depth;
wherein rotation of the output polarizer by ±π/4 with respect to the one rotation orientation for full color enhancement gives a neutral transmission spectrum, where the transmission is the same across the visible spectrum; and
wherein rotation of the output polarizer by ±π/2 with respect to the one rotation orientation for full color enhancement gives full color suppression (CS), with at least one peak in each of the cyan and yellow wavelengths and at least one minima in each of the red, green and blue wavelengths of maximum modulation depth.
2. A filter as defined in claim 1 , wherein the filtered color coordinate for a spectrally flat input in the CE, CS, and neutral states is equivalent.
3. A filter as defined in claim 2 , wherein equivalent includes the filtered color coordinate in each state being within a just noticeable difference.
4. A filter as defined in claim 1 , wherein the filtered luminance for a spectrally flat input in the CE, CS, and neutral states is equivalent.
5. A filter as defined in claim 1 , wherein modulation of the output polarizer orientation produces two or more distinct color filtered versions of an input image.
6. A filter as defined in claim 1 , wherein synchronous modulation of the output polarizer orientation with a field rate of an electronic image capture device produces two or more filtered versions of an input for each captured field.
7. A filter as defined in claim 1 , further including a first mechanical tuning mechanism and a second mechanical tuning mechanism, wherein the first mechanical tuning mechanism rotates the two polarizers in unison, and wherein the second mechanical tuning mechanism rotates the output polarizer relative to the input polarizer.
8. A filter as defined in claim 7 , wherein the filter is affixed to a digital SLR camera and the first and second mechanical tuning mechanisms are rings.
9. A filter as defined in claim 7 , wherein the filter is affixed to a digital camera and the first and second mechanical tuning mechanisms are rings.
10. A filter as defined in claim 1 , wherein the filter provides a variable color temperature.
11. A filter as defined in claim 1 , wherein the light passing through the input polarizer comes from an illuminated scene and light passing through the filter is provided to an optical detection medium.
12. A filter as defined in claim 11 , wherein the optical detection medium is a camera sensor array.
13. A filter as defined in claim 11 , wherein the optical detection medium is a human eye.
14. A filter as defined in claim 11 , wherein the light passing through the input polarizer is light with a particular spectral power distribution and the detection medium is a color sensitive detector.