Retinal camera filter for macular pigment measurements

Abstract

Apparatus for use in measuring the density and spatial distribution of macular pigment in an eye comprises a camera ( 4 ) for capturing a colour image of an eye, at least one filter ( 36 ) for filtering light reaching the camera (preferably by filtering the light illuminating the eye). The filter ( 36 ) has a transmission spectrum with one peak in the region of light absorbed by the pigment and another peak in a region where no such absorption occurs. The filter increases the sensitivity of the camera to macular pigment whilst enabling the effect of other pigments to be reduced or eliminated. A method of measuring macular pigment involves obtaining a colour image of an eye, the image having two colour components each having a spectrum having a respective one of said peaks. Corresponding portions of the components are mathematically combined so as to provide a measurement of macular pigment density and the results of the combination are used to provide an output representative of the contribution of macular pigment to the image.

1 . Apparatus for use in measuring the density and spatial distribution of macular pigment in an eye, the apparatus comprising a camera for capturing a colour image of the retina of an eye under examination, at least one filter for filtering light reaching the camera, said at least one filter having a transmission spectrum which has a peak in the region of the wavelength of light absorbed by the pigment and another peak in a region at which no such absorption occurs. 
   
   
       2 . Apparatus according to  claim 1 , in which said at least one filter has a transmission spectrum which is substantially zero between said two peaks. 
   
   
       3 . Apparatus according to  claim 2 , in which said transmission spectrum does not exceed 0.001% between said peaks. 
   
   
       4 . Apparatus according to  claim 1 , in which each peak is no more than 40 nm wide. 
   
   
       5 . Apparatus according to  claim 1 , in which the filter is situated in front of an illuminating light source, so that the spectrum of light which illuminates an eye under examination has said peaks 
   
   
       6 . Apparatus according to  claim 1 , in which at least one filter is a single filter having both of said peaks in its transmission spectrum. 
   
   
       7 . Apparatus according to  claim 1 , in which one of said peaks is at a wavelength corresponding to blue light, the other at a wavelength corresponding to red light. 
   
   
       8 . Apparatus according to  claim 7 , in which said first peak is at 460 nm, and said second peak is at 600 nm. 
   
   
       9 . Apparatus according to  claim 7 , in which the filter is a triple bandpass filter, the transmission spectrum of which has a further peak at a wavelength corresponding to green light. 
   
   
       10 . Apparatus for use in the measuring of the density and spatial distribution of macular pigment in an eye under examination, the apparatus comprising an illumination device for illuminating said eye and a camera for capturing a colour image of the eye, when so illuminated, wherein the illumination device is operable to illuminate the eye with light the spectrum of which has a first peak at a wavelength of light which is absorbed by the macular pigment and a second peak at a wavelength at which substantially no such absorption occurs. 
   
   
       11 . Apparatus according to  claim 10 , in which the spectrum of said illuminating light falls to substantially zero between these two peaks. 
   
   
       12 . Apparatus according to  claim 10 , in which the illuminating device comprises a light source and a filter having both said peaks in its transmission spectrum. 
   
   
       13 . Apparatus according to  claim 10 , in which one of said peaks is at the wavelength corresponding to blue light, the other at a wavelength corresponding to red light. 
   
   
       14 . Apparatus according to  claim 13 , in which said first peak is at 460 nm, the second at 600 nm. 
   
   
       15 . Apparatus according to  claim 12 , in which the filter is a triple bandpass filter, the transmission spectrum of which has a further peak at a wavelength corresponding to green light. 
   
   
       16 . Apparatus according to  claim 10 , in which the apparatus includes an image processor for processing the image captured by the camera, wherein the image processor is programmed to subtract the reference component of the image from the component in the absorption spectrum of the macular pigment, thereby to remove the contribution to the image of pigments other than the macular pigment. 
   
   
       17 . Apparatus according to  claim 16 , in which the processor is operable to generate an output signal in which the results of the subtraction are represented as a macular pigment map. 
   
   
       18 . Apparatus according to  claim 16 , in which said subtraction is of the logs of the intensities of the two components. 
   
   
       19 . Apparatus according to  claim 15 , in which the image processor is operable to take the logs of three images, each corresponding to a respective peak of the triple bandpass filter's transmission spectrum, and to combine these so as to eliminate any contributions from non uniform distributions of both melanin and photopigments. 
   
   
       20 . A method of measuring macular pigment density and spatial distribution in an eye, the method comprising the steps of,
 a) capturing a colour image of the retina of the eye, the image having a first and second colour component, the first colour component having a spectrum the peak of which is at a wavelength at which the absorption by macular pigment is at a maximum and a second peak at which substantially no such absorption occurs;   b) mathematically combining the image components at each region of the image, to remove at least some colour contributions not arising from the macular pigment and   c) providing an output representative of the contribution of the macular pigment to the image.   
   
   
       21 . A method according to  claim 20 , in which the step of capturing the image involves illuminating the eye with light, the spectrum of which has said first and second peaks. 
   
   
       22 . A method according to  claim 20 , in which the image is captured by means of a camera and a filter which has a first and second peak its transmission spectrum, corresponding to the two peaks of the components, and which filters the light forming the image captured by the camera. 
   
   
       23 . A method according to  claim 22 , in which the filter is situated in the path of light from a source of illumination to the eye. 
   
   
       24 . A method according to  claim 20 , in which said step of mathematically combining the components comprises taking the logarithms of the intensities of the component at each said regions, and subtracting one logarithm from the other. 
   
   
       25 . A method according to  claim 20 , in which said image has three colour components, and said mathematical combination comprises mathematically combining logarithms for three components in a linear fashion with weighting factors applied to at least one logarithm, so as to eliminate contributions from non uniform distributions of melanin and cone photopigments over the retina. 
   
   
       26 . A method according to  claim 25 , in which a further image component of the retina is obtained at a fourth wavelength of light, and data on the further image is used in said mathematical combination to eliminate the effect of non uniform distribution of rod photopigment across the retina. 
   
   
       27 . A method according to  claim 26 , in which the further image component is obtained by capturing a further image of the retina. 
   
   
       28 . A method according to  claim 27 , in which the peaks of the spectra of the components are substantially at 460 nm (blue), 530 nm (green) 600 nm (red) and 680 nm (far red) respectively. 
   
   
       29 . A method according to  claim 28  in which the logarithms are combined to obtain the macular pigment density, D, at a given location by the formula:
     D=− 0.525 *LD   B +0.355 *LD   G −0.882 *LD   R +2.60 *LD   R′     
     where LD B , LD G , LD R  and LD R′  are the logarithms of the differences between the intensities at the given location and a peripheral location in the blue, green, red and far red component respectively.