Exposure Meter

This invention relates to an exposure meter for use by photographers and arose with a view to providing an indication of the distribution of luminance values over a field of view, rather than just the luminance of one particular part of the field of view.

We did endeavor to meet this requirement in the past using a somewhat unconventional arrangement described in our Patent Specification GB No. 693443. This displays a graph of brightness vs. horizontal angle of view. The value of the horizontal angle of view is however of no value to the user of the instrument and the brightness values, being only average values for vertical strips of the field of view, are of limited value since they could include bright areas of sky and dark shadows on the ground.

A conventional spot exposure meter has a narrow field of view compared with a scene to be photographed and provides a single output measurement representing the luminance of one particular part of the scene. In order to obtain an indication of the contrast of the scene the conventional spot exposure meter must be pointed in successive different directions and the different measurements obtained must be noted by the photographer. In a more sophisticated known system the exposure meter has a wider field of view covering the whole of a scene to be photographed and includes six photocells which provide respective luminance values for six shaped sectors of the field of view. A decision on the required exposure is made automatically on the basis of the six measured values, knowledge of the type of scene (e.g., portrait, outdoor daylight scene etc.) which is entered manually by the photographer, and various assumptions concerning the likely content (e.g. sky) and photographic importance of features in the different sectors. This automatic system is recognized as suffering from certain drawbacks. For example it does not allow for unusual types of scene or for the achievement of special dramatic or unusual photographic effects. For these reasons a special switch is provided to prevent the above automatic system from operating when not required.

This invention provides an exposure meter comprising: means for deriving different values defining the brightness of different parts of a field of view; means defining a number of cells into which the said brightness values may fall; and means for providing measures of the number of said brightness values in each cell.

The measures of the number of brightness values in each cell are preferably displayed to the operator thereby giving him a useful indication both of the overall luminance and of the distribution of luminance over the field of view. The display is preferably in the form of a histogram since this allows the information to be reasily assimilated by the photographer. A similar display is preferably arranged alongside the histogram, showing the range of luminance values which can be handled by the particular film in use at the current aperture and exposure settings of a camera. A facility has to be provided of course for details of the type of film and aperture and exposure settings to be entered either automatically or manually.

The sole FIGURE is a schematic block diagram of an exposure meter constructed in accordance with the invention.

The illustrated exposure meter comprises a lens 1 which focuses a scene onto a color camera pick up 2; Philips NXA 1020 and associated chips being used in this particular example. This includes a photosensitive plate carrying an array of photosensitive elements arranged in groups of three, one for each color, yellow, green and magenta. An electronic drive circuit 2A is associated with the chip 2 for electronically addressing each element appropriate to each color in turn so as to scan the scene focussed onto the plate. Elements 2 and 2A in combination constitute a color video camera which produces three analog video signals 3, 4 and 5 for yellow, green and magenta respectively. There is a further signal on line 2B which is known as "frame start". The outputs on lines 3, 4 and 5 are passed to respective sample and hold circuits 6, 7 and 8 and then to analogue to digital convertors 9, 10 and 11. The A/D convertors are not binary. They use a special code where one digit represents a ratio of 1/3 of one aperture value. Numerically this is equal to .sqroot.2/6.

The resulting digital signals, which will be referred to as y, g, and m signals, are presented to a processor P (RCA 1802 being used in this example) via its parallel interface ports. The processor also receives the frame start signal as an interrupt. The processor causes a keyboard 12 to be scanned in order to see which, if any, of its keys M, +, -, C, are pressed.

The processor produces in response to the C button, via a suitable interface, three new output signals on lines 13, 14 and 15 which produce a display on a display device 16 which in this example is an LCD display. The signal on line 13 produces a histogram 17. The signal on line 14 defines the position across the display and the length of a bar 18 and the signals on line 15 control numeric data displayed namely; FS value (film speed in ASA units); SS value (shutter speed in seconds or fractions of a second); AV value (aperture value in ratios from F1 to F1:8); CR value (contrast range from 2 to b 16,000:1) and M/S value (a measure of the departure from the color temperature measured from 6000.degree. K. expressed in Mireds). The processor also generates a flash trigger signal on line 19 in response to pressing the C button.

The processor has a stored program which was written in a high level language via a microprocessor development system which compiles the program into machine code, the latter being stored in an EPROM forming part of the processor. The effect of the program is to produce the signals on lines 13, 14, 15 and 19 as follows.

To produce the histogram signal on line 13 the processor converts the digital y, g, and m signals into corresponding Red, Green and Blue signals in accordance with rules known in the art. These are added and divided by three to form an average. Each average value thus obtained represents the light intensity on a group of three y, g, m, color sensors in the pick up 2. The average values are now sorted in cells where each cell spans a range of intensity values of .sqroot.2/6. There are forty-eight of these cells and the number of values in each cell are displayed on the histogram 17. The histogram display is rectangular and is in fact a dot matrix of forty eight horizontal elements and thirty six vertical elements so that the width of the display corresponds to 48/3 aperture values. This amounts to a contrast ratio of 16,000:1.

To produce the bar 18, which has a maximum potential length of forty eight dots and is one dot high, the processor first calculates the length as follows, using a contrast range value which is inserted by the user, with the + and - buttons on the keyboard 12, and which is displayed on the numeric display at CR. This value is selected by the user from information provided by the manufacture of the sensitive material being used in the camera. The length of the bar in dots is the (CR value expressed as a power of 2).times.3. The processor now calculates the position of the center of the bar. It does this using an equivalent light value (LV) which it calculates by multiplying SS by AV (to be described later). The position of the center vis-a-vis the width of the display is the product of LV and FS (also to be described later) in units defined by International Standards Organisation.

FS is a value which is inserted by the user. It is the ASA rating of the film speed of the film in the camera. The user inserts this using the + and - keys. In this connection it is explained that operation of the + and - keys adjusts one of the numeric variables marked by a cursor which is moved by successive pressure of the M button on the keyboard 12.

SS is a numeric variable inserted by the operator (again using the + and - keys) and corresponds to the shutter speed to which the camera shutter is set or to be set.

AV is the aperture value which is inserted by the user (again using the + and - keys) and corresponds to the aperture vallue to which the diaphragm of the camera is set or to be set.

The value of M/S is calculated by the processor by taking the average of all the calculated red values over one frame (determined by the frame start signal) and dividing it by the average of all the blue values and selecting a fraction (the M/S value) which when multiplied by the average red value results in the result of divisions being 1 when the pick-up chip is exposed to a source of illumination whose color temperature is 6000.degree. K. When the chip is exposed to a source of illumination of 6100.degree. K. the M/S value is -100 and when the source is 5900.degree. K. the M/S value is +100.

It is believed that the illustrated exposure meter offers the user much more information than any other meter and still allows the user of artistic tone distortion if that is what is required. On the other hand, precise exposure determination is made much simpler to achieve. There has been no attempt to automate the whole system although the microprocessor could be used to alter the shutter speed or the aperture value in order to give aperture or shutter speed priority. In this style of meter it is felt that this would be a retrograde step.