Densitometers measure the amount of transmitted light. Most are calibrated to some type of national standard and must be checked periodically to ensure accuracy. An erratic reading on your densitometer usually indicates a need for recalibration. Manual densitometers require two types of calibration to correct instrument optical drift due to changes in temperature and humidity.
Null calibration is a quick procedure that should be accomplished every time the instrument is turned on. The operator simply presses and holds the "READ" button and then presses the "NULL" button to zero out the instrument. Do not null the unit again without turning the unit off and then on again.
Periodically, the operator should perform slope calibration, which uses the instrument's supplied calibration plaque to check a known density to what the instrument records. The operator reads the "CAL STEP" on the calibration wedge and compares the known density to what the densitometer reads. If a correction of plus or minus .02 is needed, the operator should take the screwdriver included with the densitometer and turn the calibration screw, while holding the "READ" button, to bring the densitometer back into calibration.
The calibration strip supplied with the densitometer expires in eighteen months. It may be possible to order the calibration strip and perform the test above to monitor calibration of the densitometer.
FDA prefers the facility get a loaner if recalibration is necessary. Remember, No densitometer, no processing.
A sensitometer is a light-emitting device that exposes a 21-step density pattern on film. A piece of unprocessed film is then sensitized and inserted into an automatic processor. This results in a gray scale pattern with 21 different steps.
The batteries in the sensitometers are usually alkaline batteries. These need to be replaced at least once a year or 10,000 exposures. Some sensitometers have an audible tone when the battery is low.
Unlike the densitometer, these do not have a North American Standard. To check your sensitometer repeatability, you can sensitize several strips and measure the same steps to see if it is accurate. If you suspect it is not accurate, contact the manufacturer and return the unit for recalibration. A variance in step greater than .05 would indicate that a recalibration is necessary.X-rite recommends that sensitometers be calibrated at least every eighteen (18) months.
Conducting the daily processor QC when the sensitometer is not available.
On 10-18-1999, FDA approved a request for an alternative to sensitometric-densitometric testing of processor performance that can be used for a period of up to 2 weeks when the facility’s sensitometer is unavailable.
When using the alternative test, processor performance is considered satisfactory if:
1. The optical density of the film at the center of an image of a standard FDA-accepted phantom is at least 1.20 when exposed under typical clinical conditions.
2. The optical density of the film at the center of the phantom image changes no more than +/- 0.20 from the established operating level.
3. The density difference between the background of the phantom and an added test object, used to assess image contrast, is measured and does not vary by more than +/- 0.05 from the established operating level.
4. To evaluate base+fog an additional measurement of density must be made either of a shielded portion of the phantom image film or of an unexposed film. In accordance with 21 CFR 900.12 (e)(1)(i), the base+fog density must be within +0.03 of the established operating level.
As with the original test, this alternative test must be conducted “each day clinical films are processed, but before processing of clinical films.” All results must be recorded. Again, as with the original test, if processor performance fails to meet any part of the alternative test, the problem must be corrected before processing is resumed.