318 Denistometer
First, some info on calibrating the 318...
All you need is a calibration reference from ANY reflection densitometer (color) that would have CMY&K patches. You are normalizing the system to known values. So if X-Rite does not support a 16 year old or older instrument anymore, don't blame them. THey cannot get the componenets to fix them. Ever try to find a vacume tube lately? Ask for a calibration reference for a current unit. Don't be surprised to pay some good money for one. Check their web site. For the proper reference, see below about responses.
Each channel has a high and a low. The low value can be set to the white reference, usually about 0.10 to 0.15. THe color patches are used to set the high ends. Once this is done it is a relative measure for that calibration reference.
If you can clean up the battery compartment, the NiCd battery will be useful. If holds the calibration information in memory when you turn the unit off. Otherwise you need to do a full calibration each time.
The densitometer has Red Green and Blue filters for color and a Visual response filter for B&W. The RGB filters are used to measure Cyan, Magenta and Yellow samples, respectively. It is important to understand this because on some densitometers the filters are labeled RGBV and on others they are labeled CMYK. Usually, reflection denisitometers marketd to the printing industry named the filters CMYK. The photo industry used RGBV. They were still RGB and V filters in the units. However, for the nees of the printing industry and photo industry there are differences.
The filters are slightly different in their responses, the amounts of light they absorb or let pass. Somewhere on the densitometer it might state what response (filter set) the unit has. Responses T or G are for the graphic arts industry. Response A is for the photo industry. Response M was only found in transmission densitometers for the photo industry. If the values stated say CMYK, you probably have a unit designed for the graphic arts. You cannot compare data with someone else using a unit designed for the photo industry, but for your own purposes, it should work fine. The biggest difference will be in the Blue filter (Yellow channel). The Blue filter in the graphic arts units had a wider spectral response (saw more light in the blue portion of the spectrum) than the photo unit filters did. This will result in a lower value to the Yellow channel.
I stated the values on the densitometer will be relative. This is due to the unknown condition of the filters. Filters fade and change over time. They can get dirty and depending how the unit was stored, can even get fungus on them. They were probably gelatin on acetate filters. The gelatin is a great medium for growing fungus under the right conditions. If you can open the unit up (make sure you can get it back together), you should be able to check the condition of the filters. If they look cloudy, they have fungus.
Second- RGB vs CMYK, I pretty much covered this under the part on reponses. Modern densitometers no longer use the RGBV filters. They use thirty two of them and actually measure the spectrum. This data is integrated mathematically to give density values for the different responses. The nice thing about this is a densitometer can be both photo (Status A) and graphic arts (now either Status T, E or I) or anything in between. The measured data is just weighted differently for the response chosen on the instrument. The only bad thing for the user, if the manufacturer discontinues the filter wheel, you are out of luck if you need the unit fixed. The filters are special "narrow band interferance filters" that see a very small portion of the spectrum,about 10 nm. They are not cheap and you cannot get them from just anywhere.
Finally, if you are trying to compare visual perception to denity values, be careful. Going from matte to glossy there will be big differences in the measured values. It should correlate to visual impression, but will not track it perfectly. This is due to the design of the optics, still in use today. A matte sample will not be as dense, nor have as many steps as a glossy print/sample. The contrast of the glossy sample (in densitometry terms, this is called slope or gamma) will also be higher. This is caused by the visual effect of the matte paper. It scatters more light, thereby reducing both contrast and effectively reduces maximum density (DMax).
If you need more info, let me know.
Enjoy the unit!
DA
