GB2299688A - Darkroom analyser - Google Patents

Abstract

A darkroom analyser has a main control box 10 and sensor box 12 with sensor 32 which is placed under an image in an having means for enlarger to sense the intensity of the light at that point of the image to be projected onto printing paper and by use of a precalibrated scale displays on display 16 the print density of that point on the resulting print after exposure and development of the printing paper under the same parameters as employed during calibration. The analyser can also control the subsequent printing of the enlarger.

Description

Darkroom Analyser This invention relates to a light meter for use in a photographic darkroom.

According to the present invention, there is provided a darkroom light meter having means for sensing the intensity of the light at a point in an image to be projected onto printing paper and a precalibrated scale for providing a visual representation of the print density of that point on the resulting print after exposure and development of the printing paper under the same parameters as employed during calibration.

The light meter of the preferred embodiment of the invention enables the user to visualise accurately the effect of exposure and contrast control upon the intended print. The calibrated scale permits print-zone indication of one or more meter readings of the enlarger image.

Preferably, the light meter incorporates a safelight control to switch the safelight off automatically during the measurement.

Advantageously, the light meter also includes a timer to control the switching of the enlarger lamp. The timer may suitably include an initial time delay to allow physical vibrations of the enlarging equipment to settle before switching on the enlarger lamp, thereby preventing image degradation through enlarger shake.

In use, several spot readings are taken from the same image to ensure that both the highlights and the shadows in the image are rendered at the correct print density. Should the range of the tones be narrower than desired, then a different contrast grade of paper may be tested by selecting a different set of calibration values.

It is desirable to use a micro-computer to store the calibration values and to use a set of light emitting diodes to indicate the scale. The use of such a scale permits instant visualisation of the range of tones in the finished print and one may alter the exposure time to move the entire range across the scale and one may select the appropriate contrast grade of paper to expand and contract the tonal range to optimise the desired visual effect.

The advantages offered by the light meter of the preferred embodiment of the invention include the following: Previsualisation of several spot meter readings from image using print zone scale indicator, re-evaluated for different exposures and contrast settings.

Display of relative image intensities in logarithmic units.

Automatic control of safelight during meter reading.

Automatic enlarger shake control delay on printing.

Automatic fine adjustment test strip maker.

Automatic meter sensor settling for low intensity images Equal Exposure steps.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows an enlarger analyser of the invention, Figure 2 is a block diagram of the analyser shown in Figure 1, and Figure 3 is a flow chart showing the operation of the analyser in Figures 1 and 2.

Detailed description of the preferred ernbodiment The darkroom analyser in the drawings is designed for use with monochrome enlargers, using fixed and variable contrast papers. The analyser has a memory with pre-calibrated exposure information for popular papers, and may be adjusted to suit personal taste.

The analyser consists of a main control box 10 and a remote sensor box 12, the contents of which are shown in Figure 2.

The sensor box is designed to measure a small spot (5mm diameter) of the projected image.

The main control box 10 has an alpha numeric display 16 on which grade and exposure information as displayed and controls the enlarger bulb and safe-light. A key pad 18 with eight buttons, described in detail below, is provided for user input and a row of LED's 14 lying adjacent a printed grey scale allows the user to pre-visualise the density of a point on the finished print from a sensor reading.

Within the main control box, the main element is a 68HC05 microcontroller 20 having a bus 22 to allow it to communicate with the alpha numeric display 16, the print zone indication display 14 and the keypad 18 on the front panel. The control box also contains a power supply 24 connected to a mains power switching unit 26, an EEPROM 28 on which are stored the program and calibration information used by the microcontroller 20 and an interface circuit 29 that connects to the sensor box 12.

Within the sensor box 12, there is the sensor proper 32 and a button 30 that is pressed by the user when the sensor is lying on the baseboard of the enlarger at a point of the negative image to be measured.

Each time the user presses the button 30, one of the LED's of the display 14 will light up to indicate the density that this point will produce on a developed print if exposed at the prevailing settings of brightness and contrast. By taking several readings, as described in more detail below, the user can obtain an indication of the brightness and contrast range of the resulting print.

Zone Control The analyser can record and display up to eight spot measurements from the negative area, suggesting an exposure based on the median tonal value, at the selected paper contrast grade. Each exposure sample is shown as a print zone on a 15 segment bar graph adjacent to a printed grey scale. After the measurement have been taken it is possible to use the exposure and contrast control buttons to visualise the effect of overriding the suggested exposure time and changing paper grade by reference to the bar graph.

Development Control The analyser can be used as a negative densitometer at the paper surface. Each time a measurement is made, the display shows (for a short period) the negative density in relation to the brightest sample. In combination with a white/grey/black test card subject development dependent negative contrast control may be assessed.

Automatic Intelligent Test Strips The analyser has the ability to make seven test strips in 1/5th stops about the suggested or altered exposure time.

This is achieved with just one button press in association with a test strip mask.

Sensible Exposure Times The analyser calculates exposure times to the nearest 1/5th EV steps. These exposure times are repeatable and give even incremental exposure values. Exposure times are limited to 120 seconds and are controlled to the nearest 1/2 second.

Exposure times can be adjusted manually using the exposure override buttons both before and after exposure metering.

Calibration The in-built calibration suits standard variable contrast e paper, such as Ilford Multigrade , and is used as the default user calibration. However, if the user requires overall darker or lighter results to suit a different batch of paper it is possible to override the calibration.

Measurement Range The analyser has an in-built 9 stop working range, and will give out of range warnings for dark and light extremes. It will also give warnings if the required exposure falls outside the exposure time control range.

Using the Analyser Setting Up To set up the exposure analyser for work connect the safelight and enlarger mains plugs into the appropriate mains sockets on the control box. Connect the 6V DC input to the socket at the side. The display should display standard grade and print time on the alphanumeric display. The safelight should be on and the enlarger off. Removal of the 6V DC input will cause the unit to power down.

Controls There are 8 buttons on the control box: 1. + Exposure Increase Override button 2. + Exposure Decrease Override button 3. < > Contrast Increase Button 4. > < Contrast Decrease Button 5. PRT Print start button 6. LGT Enlarger on/off toggle Button 7. CLR Clear measurements/test strip/calibration 8. CAL Calibration button 9. Measuring Head button This increases exposure by one fifth stop before or after making an exposure measurement.

+ This decreases exposure by one fifth stop before or after making an exposure measurement.

This changes the tonal and exposure values to suit a higher contrast paper > < This changes the tonal and exposure values to suit a lower contrast paper PRT This will do one of two things: a) With the enlarger off it will make a print exposure of the displayed value.

b) With the enlarger on it will commence a test strip sequence, with further presses for each strip exposure.

LGT This toggles the enlarger light on and off. (Measurements can only be taken with the enlarger on.) CLR This key will do one of three things: 1.Clear any measurement values from memory, but retain the last exposure time.

2.Abort a test strip sequence in-between strip exposures.

3.Abort a calibration procedure.

4.Abort a print exposure.

CAL This key makes a simple numerical measurement for test purposes and programs in the current exposure offset.

The measuring button on the head makes a light measurement when the enlarger is on.

Understanding the Display The display shows paper grade, exposure time, density, exposure compensation, and simple messages at different times.

If the enlarger is turned on and a measurement taken from the shadow area of the negative, the display is replaced by 'BUSY'. After the measurement value has settled the display shows the negative density for a brief time and then shows the grade and the new exposure time.

If another measurement is taken, from a highlight area of the negative then the display will show the exposure for the average of the two measurements and the placement of each of the measured areas in print zones.

If a contrast change button is pressed '2' will be replaced by '3' or '1', and the exposure time will change to give the same mid tone. In addition the zone indicators will change to show the effect of changing paper grade.

Exposure Metering Turn the enlarger on with the LGT button and position and focus the negative. Ensure that there are no filters in the light path. Using the Contrast Change buttons, select the grade of paper you want to use. Press the CLR button to erase any previously stored light values.

Move the measuring head to the shadow area of the image and press the measuring button. After a small delay, shown by the message 'BUSY' on the display, a suggested exposure time will be displayed after 'DO.OO'.

Move the measuring head to a highlight in the image and press the measuring button. After a longer delay (allowing the light sensor to react to the very small light level ), the density range in logarithmic units is shown temporarily followed by the new exposure time and two print zones.

If either the extreme left or right hand LED's flash, then the print will have pure white and black tones respectively.

If both outer LED's flash on the bargraph then the density range is too great for the paper grade you have chosen for highlight and shadow detail to appear in the final print together. By choosing a lower contrast paper it may be possible to accommodate the wide density range.

Up to eight separate reading may be taken, with the overall exposure being based on the mean exposure of the extreme light values. The various intermediate zones will be displayed on the bar-graph against the grey scale. This can be useful in confirming highlight, shadow and skin tone print zone placement. If more than eight readings are attempted the last measurement will be ignored.

Pr Printing With the enlarger off, position the paper in the easel select (and insert the indicated contrast control filter and press the PRT button. There is a 4 second delay for vibrations to die down and then the enlarger will expose the paper for the displayed time.

The exposure time counts down to zero in order to facilitate dodging or burning in exercises.

The printing may be cancelled by pressing the CLR button.

Adjusting the Desired Result Once a set of measurements have been taken, and even after making a first print it is possible to simply modify the contrast and exposure settings to achieve the desired result. Until the CLR button is pressed one can pick up on the metering task consolidating on the previous measurements.

Changing the exposure time will shift all the print zones darker or lighter in 1/5th stop steps. The exposure adjustment is shown on the display for a short time. The display then shows a new print time calculated to the nearest quarter second.

When a print zone lies outside the limits of the paper range a flashing LED will be displayed to show excessive under/over exposure.

Changing the contrast will display a new set of print zones based on the measurements you have taken.

(If you find yourself using a consistent exposure adjustment then it is handy to write it on the paper box and use the calibration procedure.) Test Strips After making a judgement of the required exposure or by taking measurements, turn the enlarger on and press the PRT button. The grade display is replaced by the message 't' and the enlarger light will extinguish. Place the test strip mask in place with the paper and uncover all the strips. Press PRT again to expose. Cover one strip ONLY and press the PRT button for the second exposure. This procedure is repeated 7 times, each time the total exposure time is incremented by 1/5th of a stop. After the last exposure the normal display and mode is resumed. Pressing CLR at any time should abort this test strip sequence.

Calibration This feature can be used to override the calibrations held in memory. There is one calibration adjustment, based on 1/5th stop steps to make all prints darker or lighter. It is easy to clear any changes made and is useful to write the calibration value onto the box of paper currently in use.

After making a measurement, with the print zones still displayed, use the exposure up/down buttons until the desired adjustment flashes on the display. Try pressing twice.

Press the CAL button and the gauge will make a measurement (used for test purposes only). It will display a number e.g. 'C 128' which can be ignored. Soon after the display will show 'e 2' . This has now programmed a +2/5ths of a stop adjustment to all measurements.

To clear the calibration simply press the CAL button again immediately, or after pressing CLR.

Claims (7)

1. A darkroom light meter having means for sensing the intensity of the light at a point in an image to be projected onto printing paper and a precalibrated scale for providing a visual representation of the print density of that point on the resulting print after exposure and development of the printing paper under the same parameters as employed during calibration.

2. A darkroom light meter as claimed in Claim 1, further incorporating a safelight control switch to switch a safelight off automatically during the measurement.

3. A darkroom light meter as claimed in Claim 1 or 2, further including a timer to control the switching of an enlarger lamp.

4. A darkroom light meter as claimed in Claim 3, wherein the timer includes an initial time delay to allow physical vibrations of the enlarging equipment to settle before switching on the enlarger lamp, thereby preventing image degradation through enlarger shake.

5. A darkroom light meter as claimed in any one of Claims 1 to 4, further comprising means for memorising calibration values for different types of photographic paper.

6. A darkroom light meter as claimed in any preceding claim, wherein the visual representation of the print density of a point in the finished print is achieved by the illumination of one of a set of light emitting diodes positioned adjacent a print density scale.

7. A darkroom light meter constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.

7. A darkroom light meter constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.

Amendments to the claims have been filed as follows 1. A darkroom light meter having means for sensing the intensity of the light at one or more points in an image to be projected onto printing paper and a precalibrated scale for providing a simultaneous visual representation of the print density of each measured point on the resulting print after exposure and development of the printing paper under the same parameters as employed during calibration.

2. A darkroom light meter as claimed in Claim 1, further incorporating a safelight control switch to switch a safelight off automatically during the measurement.

3. A darkroom light meter as claimed in Claim 1 or 2, further including a timer to control the switching of an enlarger lamp.

4. A darkroom light meter as claimed in Claim 3, wherein the timer includes an initial time delay to allow physical vibrations of the enlarging equipment to settle before switching on the enlarger lamp, thereby preventing image degradation through enlarger shake.

5. A darkroom light meter as claimed in any one of Claims 1 to 4, further comprising means for memorising calibration values for different types of photographic paper.

6. A darkroom light meter as claimed in any preceding claim, wherein the visual representation of the print density of a point in the finished print is achieved by the illumination of one of a set of light emitting diodes positioned adjacent a print density scale.