Student Experiment Darkroom Fog Check and viewbox Illuminations
To ensure that the safelights and other potential sources of “unsafe light” do not fog the film being handled in the darkroom.
1. X-ray film
2. Clock or stopwatch
4. Radiographic room
6. Darkroom safelight test form from CD-ROM
1. Turn off all of the safelights and overhead lights in the darkroom.
2. After eyes are adjusted to the darkness (about 5 minutes), look for any sources of light that you can find. Pay particular attention to the seals around processors, passboxes, darkroom doors, and so on. Suspended ceilings can leak light from the surrounding rooms. Make note of any light leaks and indicate them in the analysis portion of this experiment. Turn the safelights back on.
3. Remove an 8 10 inch cassette from the passbox, and take it into one of the radiographic rooms. Place it in the center of the x-ray table at a 40-inch (100-cm) source-to-image distance (SID). Place a penetrometer in the center of the cassette, aligning the long dimension of the wedge with the long axis of the cassette. Collimate the light field to the edges of the step wedge.
4. Expose the cassette using approximately 70 kilovolts (peak) (kVp) and 5 milliamperes-second (mAs) or enough to give the center step of the step wedge pattern an optical density of approximately 1.0.
5. Return the cassette to the darkroom and remove the exposed film from the cassette. Lay it on the counter and cover one half of the film with a sheet of cardboard or other opaque material.
6. Expose the film to normal safelight conditions for 2 minutes and then process the film.
7. Place the film on a viewbox illuminator and observe if there is a defined line or break between the halves. Use a densitometer to record the optical density of each half. Determine the maximum density difference by subtracting the two optical density values for each step. The step with the greatest maximum density difference indicates the darkroom fog level. Record all data on the Safelight Test form.
Part 2 Viewbox Illuminations
To verify that the illumination levels of department viewboxes are within the recommended guidelines and that the ambient light conditions within the viewing area are conducive to proper viewing.
2. Various department viewboxes
3. Viewbox Quality Control Test Form from CD-ROM
1. Set the photometer to measure illumination in candles (cd) per square millimeter (nit). Place the aperature opening 9 inches away from the center of the viewbox illuminator and record the reading on the data page.
2. Mentally divide the view box into quadrants and take a light measurement from the center of each quadrant. Record the readings on the View Box Quality Control Test Form. Compare each of these values with each other and with that of the center obtained in Step 1. These values should be within ±10% of each other.
3. Repeat Steps 1 and 2 for the other viewboxes within your particular bank. Using the center readings, determine the maximum variation in brightness among the viewboxes in a particular bank. These values should be within 15% of each other.
4. Set the photometer to record illumination in lux or foot-candles (ft-c). Measure the ambient light levels by turning off the viewbox, but leave on all of the overhead room lights that are normally in use. Place the light meter 9 inches away from the viewbox front and record the reading (even though the viewbox is off). The maximum ambient light should be about 320 lux or 30 ft-c.
PART 1 DARKROOM FOG CHECK
1. Were there any light leaks visible from inside the darkroom when all of the lights were turned off? If so, how would you correct the problem?
2. Examine the optical density readings taken from each side of the step wedge image and comment on any differences that were observed. Was the darkroom fog level within acceptable limits? (The maximum density difference should be less than 0.05 optical density units.)
3. How would you correct your darkroom conditions if your darkroom fog level were exceeding the allowable limits?
4. What effect on radiographic quality (e.g., optical density, contrast, visibility of detail) could occur as a result of improper safelight conditions? Be specific.
PART 2 VIEWBOX ILLUMINATORS
1. How did your viewbox illuminator’s light output in the center compare with accepted guidelines (i.e., was it at least 1500 nit)?
2. How did the uniformity of light output between each quadrant compare with accepted limits ( 10%)? How could this affect image quality if it did not meet accepted variances?
3. How did the center readings of each viewbox panel within a particular bank compare with each other? Was the maximum variation within accepted limits (15%)?
4. Did the ambient light conditions in the viewing room meet accepted guidelines? How would image quality be affected if the ambient light is too great?
6. List some of the steps that could be taken to correct improper viewbox illumination and ambient light conditions.