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  1. #11

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    Before searching for an OB for your developer might I suggest you run some ra4 control strips? If you're using fresh paper and your chem. is adequate you should have a baseline for dmin to work with. The current kodak vc paper has an excellent, bright paper white. Any paper that's slightly out of date or corked chem. will show significant increases in dmin.

  2. #12
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    OBs will NOT affect archival qualities of prints. They can affect the yellowing of prints with long term exposure to sunlight due to degradation of the OB, but the OB in the developer has been selected to minimize this problem by leaving behind no yellow fragments and thus the brightening effect fades with time.

    The OB is specific to the paper and can have several functions. The wrong one will have the wrong (or no) effect.

    PE

  3. #13

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    Quote Originally Posted by mts View Post
    It has been my understanding that optical brighteners work by converting UV (in sunlight) to white light, and are therefore ordinarily used in washing detergents to yield "whiter whites." I wonder how they might be able to improve whites in photo papers that are normally viewed under artificial light with negligible UV, although there is no doubt a bit of UV emitted by florescent bulbs. If the intent is to remove residual dyes, then we aren't talking about the same sort of OBs that are used in detergents....
    Optical brighteners work by absorbing in the UV part of the spectrum and re-emiting in the blue (visible) spectrum. The re-emited blue is an increase in the total light reflected and serves to decrease the influence of yellow so that the material appears to be more blue (more blue-white). As you pointed out, if there is no UV in the incident light, then the OB doesn't do anything. However, there are many cases in which there is sufficient UV to excite the OB and provide the increased appearance of whiteness -- that's why manufactures of many different types of paper and fabrics use OBs.

    In the case of Kodak use of an OB in color developer, it is the same type of OB (stilbene derivative) as is used in detergents. Many materials have more than one function. The particular OB chosen by Kodak had the ability to help remove freed sensitizing dyes. The optical brightening effect was not wanted so an OB was chosen that performed the desired removal of sensitizing dyes but did not provide very much brightening (and most of it was washed out) so as to avoid a bluing of the blacks which was not desirable.

  4. #14

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    Quote Originally Posted by frotog View Post
    Before searching for an OB for your developer might I suggest you run some ra4 control strips? If you're using fresh paper and your chem. is adequate you should have a baseline for dmin to work with. The current kodak vc paper has an excellent, bright paper white. Any paper that's slightly out of date or corked chem. will show significant increases in dmin.
    Yes -- when I first asked my question, I had been using some Ektacolor Edge paper that may not have been stored properly, and it did have a yellowish (or actually, more greenish) tinge to the developed base. I have since been using paper that seems much better.

    As a point of curiosity, I have some almost 20 year old RC-74 paper that has always been refrigerated, that is almost as good as new, in terms of the base whiteness! I suspect that those older papers may have stored better because they were less sensitive?

    Finally - I do have some Sprint Systems optical brightener designed for use with black and white papers (although I don't think I've ever seen a b&w paper that would need this). I may try it with the RA4 paper for fun.

  5. #15
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    I have some 74 and some Plus papers that even frozen kept poorly. OTOH, I have some Endura and Supra kept at room temp and they are just fine. IDK what is up.

    PE

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