Method of Measuring the Contrast of a Holographic Projector
There is provided a method for determining an intensity ratio of a holographic replay field. The method comprises forming a holographic replay field at a plane comprising a housing, wherein the holographic replay field comprises an image area and a non-image area. The method further comprises displaying a first feature at a first position of the holographic replay field. The method further comprises measuring an intensity I A of the first feature using a first light detector, wherein the first light detector is disposed on the housing. The method further comprises displaying a second feature at a second position of the holographic replay field, wherein the second feature is substantially identical to the first feature and forms a conjugate at the position of a second light detector. The second light detector is disposed on the housing. The method further comprises measuring an intensity I B′ of the conjugate image of the second feature using the second light detector. The method further comprises calculating a ratio of the intensities I A , I B′ of the first feature and the conjugate image of the second feature.
1 . A method for determining an intensity ratio of a holographic replay field, the method comprising:
forming a holographic replay field at a plane comprising a housing, wherein the holographic replay field comprises an image area and a non-image area;
displaying a first feature at a first position of the holographic replay field;
measuring an intensity I A of the first feature using a first light detector, wherein the first light detector is disposed on the housing;
displaying a second feature at a second position of the holographic replay field, wherein the second feature is substantially identical to the first feature and forms a conjugate at the position of a second light detector, wherein the second light detector is disposed on the housing;
measuring an intensity I B′ of the conjugate image of the second feature using the second light detector; and
calculating a ratio of the intensities I A , I B′ of the first feature and the conjugate image of the second feature.
2 . The method as claimed in claim 1 , further comprising, between the steps of measuring the intensity I A of the first feature and displaying the second feature:
measuring an intensity I A′ using the second light detector, wherein the second light detector is aligned with an expected location of the image conjugate of the first feature; and
detecting a low signal from the second light detector, wherein a low signal is a signal having an amplitude or magnitude less than a threshold value.
3 . The method as claimed in claim 1 , wherein the ratio of the intensities I A , I B′ of the first feature and the conjugate image of the second feature is the conjugate contrast C conj of the holographic replay field.
4 . The method as claimed in claim 1 , wherein the step of calculating the ratio of the intensities I A , I B′ of the first feature and the conjugate image of the second feature is found by: C conj =(I A −I bg )/(I B′ −I bg ), where I bg is the intensity of the background noise of the holographic replay field.
5 . The method as claimed in claim 1 , wherein at least one of the first or second position is in the non-image area, optionally wherein both the first and second positions are in the non-image area.
6 . The method as claimed in claim 1 , wherein the second position is determined by:
a) measuring the intensity of the holographic replay field in a first and second direction;
b) determining the peak intensities in each of the first and second directions; and
c) deriving the second position from the measured peak in the first and second directions.
7 . The method as claimed in claim 6 , wherein the intensity of step a is measured in proximity of the expected location where the conjugate should appear.
8 . The method as claimed in claim 1 , wherein the expected location of the image conjugate of the first feature is symmetrical about the DC spot with respect to the location of the first feature.
9 . The method as claimed in claim 1 , wherein at least one of the light detectors is static.
10 . The method as claimed in claim 1 , wherein the first light detector is aligned with the first feature.
11 . The method as claimed in claim 1 , wherein the housing comprises a transmission area substantially aligned with the image area and a non-transmission area substantially aligned with the non-image area.
12 . The method as claimed in claim 1 , wherein the intensity measured by the first and/or second light detector is additionally used to monitor the intensity of the colour balance of the holographic replay field and/or calibrate the reference voltage VCOM of a liquid crystal device used to form the holographic replay field and/or optimise gamma of the liquid crystal device used to form the holographic replay field.
13 . The method as claimed in claim 1 , wherein the method is performed during end-of-line calibration of a device forming the holographic replay field.
14 . The method as claimed in any of claim 1 , wherein the method is performed during run-time of a device forming the holographic replay field.
15 . The method as claimed in claim 13 , wherein the device is a holographic projector, optionally wherein the holographic projector is for a head-up display.
16 . A holographic projection system comprising:
a light source arranged to output light;
a housing
a display device arranged to display a diffractive pattern comprising a hologram of a target image and to receive light from the light source and output spatially modulated light in accordance with the diffractive pattern to form a holographic replay field of the target image at a replay plane, wherein the housing is located at the replay plane, wherein the holographic replay field comprises an image area and a non-image area, the display device arranged to display a first feature at a first position of the holographic replay field;
a first light detector disposed on the housing;
a second light detector disposed on the housing; and
a processor arranged to:
measure an intensity la of the first feature using the first light detector;
display a second feature at a second position of the holographic replay field, wherein the second feature is substantially identical to the first feature and forms a conjugate at the position of the second light detector;
measure an intensity I B′ of the conjugate image of the second feature using the second light detector; and
calculate a ratio of the intensities I A , I B′ of the first feature and the conjugate image of the second feature.
17 . The holographic projection system as claimed in claim 16 , wherein the processor is further arranged, between the steps of measuring the intensity I A of the first feature and displaying the second feature, to:
measure an intensity I A using the second light detector, wherein the second light detector is aligned with an expected location of the image conjugate of the first feature; and
detect a low signal from the second light detector, wherein a low signal is a signal having an amplitude or magnitude less than a threshold value.
18 . The holographic projection system as claimed in claim 16 , wherein at least one of the first or second position is in the non-image area, optionally wherein both the first and second positions are in the non-image area.
19 . A picture generating unit comprising the holographic projection system of claim 16 .
20 . A head-up display comprising the picture generating unit of claim 19 .