GB2302412A - Intelligent optical filter - Google Patents

Abstract

The invention relates to an optical and electronic instrument which will selectively block the transmission of light to the human eye or other optical instrument. Where the intensity of part of the image exceeds a threshold value, as determined by a multi-element sensor 11 and lens 6, then appropriate elements in a filter or filters, 2 and 3, will be changed by signals from a processing unit 8, from being transparent to being partially or completely opaque. This produces an image in which no part has an intensity greater than a pre-determined value. Such a filter may be incorporated into sunglasses, driving glasses, optical instruments, ophthlalmological devices, night vision instruments, or safety glasses.

Description

Title Intelligent Optical Filter Field The invention relates to an optical and electronic instrument which will selectively block the transmission of light to the human eye or other optical instrument.

Where the intensity of light in an image would be greater than a predetermined value then it is blocked. Light whose intensity in the image is less than the threshold value is transmitted unimpeded. This has the effect of producing images on the retina of the eye, or at the focal plane of some other optical instrument, where no part of the image exceeds a predetermined intensity.

History In this field it is already known that it is possible to reduce the intensity of an image by interposing a darkened filter between the source and the eye or optical instrument as, for example, with normal sunglasses or a filter for a camera lens.

But this has the following disadvantages: (1) The reduction in intensity is applied to the whole image.

(2) The reduction in intensity is applied irrespective of the intensity of the light source.

Some attempts have been made to overcome these disadvantages by the following means: (a) There can be a gradation in the filtering characteristics so that, for example, the upper part of the image is darkened most - this usually corresponding to the sky in the case of sunglasses.

(b) The filter may be made of a material whose opacity is roughly proportional to the intensity of the light to which it is exposed - e.g. photochromatic lenses.

However, both these modifications still have disadvantages. In (a) the gradation assumes a certain spatial distribution of light intensity which is often not the case in practice e.g. dazzle from car headlights when driving at night or dazzle from the sun when it is low in the sky. In (b) the intensity of the whole image is affected by the interposed filter rather than just the brightest parts of the image. Another disadvantage in (b) is that such photochromatic lenses are relatively slow to respond to changes in the light intensity.

The primary disadvantage, which is that changes in the filtering characteristics are applied to the whole lens, occurs because with all of the existing arrangements there is no means of anticipating which area of the filter will lie directly between a'light source and the image of the light source on the retina or the source's image in the focal plane of an optical instrument.

STATEMENT OF INVENTION The present invention provides an optical electronic instrument which, in use, is interposed between a light source and an optical device, which may be the human eye, to limit the intensity of light reaching the optical device; comprising at least one liquid crystal device adapted to be mounted between the optical device and the light source, the liquid crystal device having an array of elements the transmissivity of which can be altered electrically, a photoelectric imaging means adjacent the liquid crystal device and exposed to the light source, and signal processing means responsive to image signals from the photoelectric imaging means representing light intensity greater than a predetermined value to reduce the transmissivity of selected elements of the liquid crystal device.

The liquid crystal device will typically have a matrix array of pixel elements.

The imaging means preferably comprises a charge coupled device and a focusing lens.

The charge coupled device and focusing lens may be combined with the signal processing means to form a single semiconductor integrated circuit.

Preferably the charge coupled device consists of an array of light sensitive elements the voltage of each being proportional to the intensity of a light imaged onto them.

Preferably, a mapping exists between each element of the charged coupled device and the pixels of the lcd screens.

Typically, the processing unit allocates a voltage to each pixel dependent on the intensity of light registered at the corresponding element of the charged coupled device relative to the average intensity registered by all the elements, the absolute value of the intensity at any element, and the duration of any intensity greater than a predetermined value.

The pixels may be affected by an occlusion pattern in the form of one or more of the following: (i) a single pixel or several adjacent pixels being partially or completely darkened; (ii) a pattern of opaque and transmissive pixels formed to allow the transmission of a reduced intensity of light; (iii) a single pixel of several adjacent pixels having their opacity fluctuated rapidly so as to allow a reduced average intensity of light to be transmitted.

Typically, the device of the present invention may be powered by an internal or external battery and/or by one or more solar cells incorporated into the frame of the device.

The device of the present invention may be used in the manufacture of sunglasses or for use with a camera or binoculars. Alternatively, or in addition, it may be used in any situation where it is desirable to limit the light intensity incident on the retina or a piece of optical equipment.

Description While further modifications and improvements may be made without departing from the scope of this invention, the following is a description of five applications of the invention with reference to the accompanying drawing.

The device in its manifestation as intelligent sunglasses consists of a frame, 1, supporting two liquid crystal displays, 2 and 3, and also supporting, probably on the bridge of the frame, a charge coupled device, 7, and lens, 6, and possibly, but not necessarily, a processing unit, 8. The processing unit may be combined with the charge coupled device and lens as a single semiconductor integrated circuit or the processor may be remote from the frame. The liquid crystal displays (lcd) may be flat or curved, coloured or monochrome.

These displays are normally transparent but individual pixels of the display can be darkened as required.

Principle of operation.

Light emanating from a light source, S, is focused by a lens, 6, to form an image on a charge coupled device (ccd), 7, consisting of an array of light sensitive elements. The voltage of each of these elements, being proportional to the intensity of the light imaged onto them, forms the input to a processing unit, 8.

The processing which occurs in the processing device performs two functions. Firstly each element of the charge coupled device is mapped to corresponding points on the lcd screens. This mapping is a function of the geometry and relative positions of the ccd, the lod screens, and of the eyes, or other optical device. Each element of the ccd, e.g. 11, corresponds with one, or possibly a cluster, of elements or pixels, e.g. 4 and 5, in each of the liquid crystal displays, 2 and 3. The mapping from element 11 of the ccd to pixels on screens 2 and 3 is such that pixels 4 and 5 intercept light in transit between source, S, and the images, I1 and I2, of source S on the retinas, 9 and 10. Thus by applying power to pixels 4 and 5 to darken these pixels the light from source S is blocked, or partly blocked, from reaching the retinas. An initial setting-up procedure would produce the appropriate values for the variables used to accomplish this mapping.

The second part of the processing undertaken by 8 is to allocate a voltage to each lod pixel and thus control its light transmission. The relevant factors in this part of the processing are the intensity registered at each pixel of the ccd relative to the average intensity registered by all the pixels, the absolute value of the intensity at any pixel, and the time duration of intensities above a certain value.

The occlusion pattern of the pixels may take the form of: (i) a single pixel or several adjacent pixels being partly or completely darkened or (ii) of a pattern of opaque and transmissive pixels being formed to allow the transmission of a reduced intensity of light or (iii) of a single or several adjacent pixels having their opacity fluctuated rapidly so as to allow a reduced average intensity of light to be transmitted.

The device may be powered by an internal or external battery and/or by power generated by solar cells built into the frame.

In its manifestation as an intelligent filter for an optical instrument the device would be essentially the same as for the sunglasses but would only utilize a single lcd screen if used with a still or video camera.

In these cases the lod screen would be placed in close proximity to the camera's focussing lens. The charge coupled device and processing unit would be located close to the lod screen and camera lens and a suitable mapping used by the processing unit to match pixels in the ccd to pixels in the lod. In the case of binoculars two lod screens would be used, one in front of each lens.

The device could also be used as a safety filter to block the transmission of light of intensity greater than a predetermined value. Applications would include welding goggles and laser safety glasses.

The device could also be used as patient-worn eye filters in ophthalmology to help overcome certain visual defects.

The ability to vary the occlusion pattern of the pixels in the lod screens would allow both the field of view and the intensity of the image for each eye to be independently controlled.

The device could also be used as a means of preserving "night vision" in circumstances where the ambient light intensity may fluctuate. This would be useful, for example, to navigators on ships and aeroplanes where presently precautions are taken to avoid exposure to relatively bright lights in order to preserve the eye's sensitivity to dim lights once the subject has become acclimatised to the dark. The processing performed by the device in this application would be to partially occlude bright parts of an image so as to limit the light intensity incident on the retina.

Advantages The devices described have the following advantages over conventional optical filters. The reduction in image intensity can be made non-uniform throughout the image and can be made to be dependent on the intensity of the unfiltered image. Thus, for example, the brightest parts of the image can be reduced in intensity while darker parts are unaffected. Furthermore the algorithm used by the processing unit to determine the filtering characteristics can itself be dependent on the average intensity of the image and also dependent on the preferences of the user. Thus, for example, the image of the headlights of an approaching car could be left unfiltered during daylight but might be partly filtered at night. The speed of response of the device will be fast enough to ensure that bright light sources are tracked as they move within the image and the appropriate pixels are darkened on the lcd screens so that these bright light sources are eclipsed as they move within the observer's field of vision. The eye or other optical instrument can then distinguish relatively faint details in the image because they are no longer exposed to excessively bright parts of the image which would normally affect the aperture of the pupil of the eye or the f number and/or exposure time of an optical instrument or pose a safety hazard.

Claims (8)

1. An optical filter which responds to the spatial distribution of light intensity incident upon it such as to limit the maximum intensity of parts of the transmitted image.

2. A filter as claimed in Claim 1 where the incident light is imaged by a charge coupled device and focusing lens.

3. A filter as claimed in Claimsl and 2 where the output from the charge coupled device is used as the input to a processing unit.

4. A filter as claimed in Claim 3 where the output of the processing unit is used to control the opacity of pixels of a display such as a liquid crystal display.

5. An optical filter as claimed in any preceding claim wherein the processing unit maps spatial elements in the charge coupled device to certain pixels in the display.

6. An optical filter as claimed in any preceding claim wherein the threshold intensity above which light is blocked is dependent on the spatial and temporal average intensity of the image.

7. An optical fiter such as claimed in any preceding claim where the power source for the imaging, processing and display functions may be a battery or a solar cell.

8. An optical filter substantially as herein described and illustrated in the accompanying drawing.