Frequency of Calibration Test:
Under normal operating conditions, the instrument calibration test should be performed once a week.
Calibration of this instrument should remain stable over a long period of time and over an extended range of voltage and temperature. However, if calibration is questioned a calibrated step tablet (301-27) is provided to check against. Handle this tablet with care so that no dirt or fingerprints are deposited on the film surface.
Check calibration while in Density mode, in the center of the step tablet area marked "CAL. "This measurement should be within +0.02D of the value marked on the step tablet. Always return the step tablet to its protective cover.
Den Cal Procedure
If calibration needs to be changed, proceed as follows after nulling the instrument:
Select Density by pressing "DEN" button
Insert a small screwdriver into the hole on the right side of the instrument, engaging the potentiometer slot inside
Measure the calibrated step tablet in the center of the area marked "CAL."
If the reading in the display does not equal that of the Calibrated step tablet, adjust the potentiometer until it does. (To increase scale factor, rotate the screwdriver clockwise.)
Check the units null (zero) and re-null if necessary
Repeat Steps 3 and 4 until the calibration and null settings are exact:
NOTE: The instrument measures for only a few seconds after the "MEASRUE" button is pushed and held. Potentiometer changes after the bulb turns off and will not be noted until a new measurement is made.
A Method For Obtaining Practical Dot Area Measurement The geometry standards for Transmission Densitometry are the American National Standards Institute (ANSI) Ph2.19 and the International Standards Organization (ISO) 5/2. These and the other supporting standards were produced with only density in mind. When a unit adheres to these standards, it may produce dot area measurements that are not in agreement with practical dot area values seen and used in the industry. Until a new or revised standard is developed and approved for the graphic arts industry, the only proper and correct methodology is to produce densitometers to the existing standards. Therefore, in order to achieve practical dot area measurements, a special dot area calibration procedure is necessary and is described below.
Dot Calibration Procedure
Verify calibration of the unit using the calibration step tablet supplied with the unit and using the procedure outlined in den calibration procedure
Measure the dot area of a known 50% dot (i.e., UGRA plate control wedge or RIT micro-line resolution target) making sure to zero on the base of the film
Take the displayed value and subtract 50% from it. Take this new value and move the decimal point over one digit to the left (i.e., with a unit that measures to the nearest percent, a displayed value of 51% - 50% = 1%. Moving the decimal point one digit to the left produces .1)
Subtract the value obtained in step 3 above from the "cal" step value on the step tablet and write this number below the number already there and add the words "DOT" to the left of the new value. (i.e., "Cal" step value of 2.95D - .1D = 2.85D should be written on the "cal" step of the step tablet).
This procedure does understate the HIGH densities somewhat (i.e., a 4.56D Dmax would really be a 4.8D from the example above), but this is not a significant amount when dealing with Dmax. If
Density measurements traceable to a standard are required, and then recalibration to the original density stated on the step tablet will be necessary.