Deterioration in CCD sensitivity
The sensitivity of the CCD in this camera can deteriorate over time.
Because ultraviolet light contains higher energy compared to visible light, the elements making up the CCD can suffer damage; so depending on the amount of ultraviolet light energy accumulated, a lowering of the sensitivity may occur. The amount of radiated energy is equal to the amount of light radiated and the length of time it is radiated, thus the total accumulated radiated energy is the sum total of all the radiated energy accumulated over time.
The total accumulated radiated energy [J/cm2] =
Σ(Amount of radiated light [W] ⋅ length of time it is radiated [in seconds])
As an example of this particular characteristic, see the following.
Characteristic Example 1:
With ultraviolet light having a wavelength of 266 nm, and total accumulated radiated energy of 100 J/cm2, the sensitivity of the CCD will be reduced by about 1%.
As an easy guideline to calculate the duration of time which causes a 1% reduction in the sensitivity with a 226 nm ultraviolet light source (Characteristic Example 1), we will introduce a simpler version of the formula used above to calculate the total amount of accumulated radiated energy.
Time which causes a 1% reduction in the sensitivity (in years) =
3.17/ amount of radiated light [∝W]
This formula is based on the amount of radiated light under one light source, with the camera being used continuously for 24 hours a day. For example, with
1.5∝W of radiated light, which is the
Characteristic Example 2:
With ultraviolet light having a wavelength of
196 nm, and total accumulated radiated energy of 15 J/cm2, the sensitivity of the CCD will be reduced by about 15%.
As an example of the decision standard, you can think of a 15% reduction in sensitivity as a guideline to use to decide when to replace the CCD. However, even though the conditions under which you use your
Appendix
camera change (for example, how wide you open the lens, how long the exposure time is, how high the gain, etc.), it is not absolutely necessary to change the CCD as long as the desired standard of image quality continues to be produced.
As an easy guideline to calculate the duration of time which causes a 15% reduction in the sensitivity with a 196 nm ultraviolet light source (Characteristic Example 2), we will introduce a simpler version of the formula used above to calculate the total amount of accumulated radiated energy.
Time which causes a 15% reduction of the sensitivity (in years) =
0.48/ amount of radiated light [∝W]
This formula is based on the amount of radiated light under one light source, with the camera being used continuously for 24 hours a day. For example, with
0.5∝W of radiated light, which is the standard output from one light source, the number of years it would take the CCD to suffer the 15% deterioration in sensitivity necessary to require replacement is 0.96 years. So if the camera is used 8 hours a day, then the time to replacement of the CCD is three times that, or
2.88years.
This is how the sensitivity of the CCD deteriorates from the effects of the radiation of ultraviolet light. When the wavelength of the radiated ultraviolet light changes, it can be assumed that the total amount of accumulated radiated energy which causes the deterioration in the sensitivity will vary greatly. Please be aware that it is possible for the sensitivity to deteriorate in a short time, too, especially when a large amount of ultraviolet light is used. (When an ultraviolet laser is shined directly into the camera and a portion of the usable picture element area of the CCD receives a significant amount of ultraviolet light, that portion of the CCD alone can suffer from deterioration in sensitivity.)
Even when there isn’t a significant amount of light entering the camera, but the exposure to the light continues for a long time, the sensitivity can be affected. So close the shutter when not actually shooting, and protect the CCD from ultraviolet light as much as possible.
Because the CCD is replaceable, when you want to replace it due to deterioration in sensitivity, contact the store where you purchased the camera. However, you cannot replace just the CCD, you will have to replace the front panel block as a unit, at additional expense.
| |
32 | |