Cherlyn Electronics Ltd

Reflection & Transmission Densitometers - Some notes for guidance

Models R710, R720 & R730 Colour Reflection Densitometers are designed to measure the amount of ink deposited onto a suitable substrate, usually paper, by measuring the optical densities of the inks. The T500 Transmission Densitometer is designed to measure the darkness of exposed photographic films, principally to determine their suitability for creating an image onto a printing plate using light exposure.

All of these densitometers are calibrated using a secondary standard, that is a suitable calibration reference that has been produced and measured using a master densitometer. The master densitometer is itself calibrated to a recognized and traceable international standard, and the measured values it gives are recorded with the calibration reference. Calibration of your densitometer takes the form of adjusting it so that it gives the same readings when measuring the calibration reference. Accurate calibration therefore relies on the accuracy of the calibration reference, so this should be treated with care and replaced periodically.

For graphic arts applications, the opacity of an ink layer or photographic film is represented by a measure of it's optical density, or it's ability to absorb and prevent the passage of light. Density is expressed on a logarithmic scale, and darker areas have higher densities.

Reflection Calibration References

Colour reflection densitometers operate by measuring the light coming back from a printed area, after passing through a specific layer of ink.

For the R710, R720 & R730 Colour Reflection Densitometers, the calibration reference takes the form of printed and measured patches of ink for each of the four colours, Cyan, Magenta, Yellow and Black, and a designated area of the paper, for each of which the density values are provided, having been measured using a master densitometer calibrated to a recognized international standard. Each of the reference areas can be measured with the densitometer, which may then be re-calibrated as necessary to give the correct density readings.

For the above models, the numerical density values of the calibration reference patches are preset in the instrument, and re-calibration is achieved automatically by going through a calibration process in which each of the designated patches is measured in turn, as prompted. Please refer to the user manual or on-screen help for further details.

Transmission Calibration References

Transmission densitometers operate by measuring the light transmitted through the film.

For Model T500, the calibration reference takes the form of a strip of film, in which a band has been exposed to light. The band has been measured using a master densitometer which has been calibrated to a recognized international standard, and the value has been recorded with the film. This area is designated as the calibration reference. The reference area is measured with the densitometer, which is then re-calibrated as necessary to give the correct density reading.

For Model T500 the numerical density value of the calibration reference is preset and re-calibration of the instrument is then achieved automatically by going through the calibration process as prompted. Please refer to the user manual for full details.

Routine Calibration

The modern solid state detectors and circuitry used in these densitometers is very stable. However, readings can be affected by other factors, such as dust build up on the optics, mechanical damage, etc. It is therefore good practice to check the calibration of both reflection & transmission densitometers on a regular basis, record the results, and re-calibrate as necessary. Initially, this should be typically once per week, but once it has been established that there are no changes, or that the changes are very small, then the period between checks can be increased, but should not be less frequent than once per month.

ISO 9002

Accurate calibration relies on the density of the calibration reference remaining constant. Care should be taken to keep the calibration reference in the best possible condition, for example, it should always be handled carefully with clean hands, and stored in its protective envelope when not in use. Avoid exposing it to direct sunlight, high temperatures, chemicals etc., and take care not to touch the measuring reference areas.

The accuracy of the calibration reference can be affected by various factors, such as ageing processes causing yellowing of the film, scratches or other marks developing due to mechanical damage, or dirt build up on the surfaces. To maintain an accurate reference, it is generally accepted that both printed & film references should be replaced periodically - Ihara recommend at least every 2 years. References must be replaced sooner if there are any signs of damage.

Note: This information is believed to be correct and is provided in good faith. No responsibility can be accepted by Cherlyn Electronics Limited in the event of any costs incurred or losses resulting from following these guidance notes.

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