Digital BW, The Print

Chris Aitken wrote:

>Is there a real issue with measuring paper via transmission (assuming the
>paper base has a constant transmission)?
>  
>
I think there are many reasons why that would never become a practical 
solution. But that would be a longer message.

Ernst

> Chris Aitken wrote:
> 
> >Is there a real issue with measuring paper via transmission 
> (assuming 
> >the paper base has a constant transmission)?
> >  
> >
> I think there are many reasons why that would never become a 
> practical solution. But that would be a longer message.

Please, post the longer message. I am interested.

Chris


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Chris Aitken wrote:

>I have access here to a Gretag Macbeth TD932 (transmission densitometer),
>
>
>Is there a real issue with measuring paper via transmission (assuming the
>paper base has a constant transmission)?
>
>  
>
The least I would be worried about is the consistency of the paper's 
light transmission :-)

The object of the measurements  is for correcting the ink density to get 
a perceptual correct representation of the light reflecting off the 
print. It would be different for a print that is mounted on a lightbox. 
Then your transmissive densitometer could work correctly (depends on the 
design though).

There must be a significant difference between both types of 
measurement. Especially with pigment inks that are more opaque and at 
the other hand reach their Dmax in reflection measurement sooner as the 
light reflecting from the pigment layer will not diminish above a 
certain amount of ink and the light transmission will still diminish 
with more (opaque) ink laid on top. In practice this would mean an ever 
increasing density reading in transmissive measuring (till the Dmax of 
the densitometer is reached or no more ink possible) while the inklimit 
for prints is already passed. With the more transparent dye inks where 
the top of the ink layer reflects less light this will be less dramatic 
but still give too much difference between the transmissive and 
reflective readings. I doubt there's even a nice relation between the 
two that will allow you to add a compensation curve for one or more inks 
and papers. The paperbase will deflect the light transmitted so even for 
transmissive measurements you will need compensations to get figures 
comparable to film readings, the desitometer design plays a role there 
too. Less than with film there will also be a difference between 
measuring with the light source at the emulsion/ink side and with 
measuring with the light source at the film/paper side. Dotgain will 
also work out differently in reflective and transmissive density.

With signs that are lit by the lightbox at night and by sunlight at day 
the practical solution is to use two prints at both sides of the paper 
(or on white vinyl, PET etc) the one at the back reversed in print. A 
normal print for reflective light is too low in density on a lightbox, 
the extra print at the back assures the density increases for the 
transmitted lighting.  There are inkjet sign printers that have head 
arrangements at both side of PVC fabric for that purpose. I think they 
use a plain ICC profile created for reflection only at the front (but at 
6000K or higher) and transmissive one for the different fluorescent 
light at the back + a lot of editing on the profiles. I've done similar 
things on taxi toplights in silkscreen printing (no fc just spot colors) 
to compensate for the warm tungsten light inside. For example a chrome 
yellow printed on a citron yellow to get a similar yellow in both 
conditions, the Dutch railway design dept is that demanding.

Ernst