Re: Measuring density
Re: Measuring density
- Subject: Re: Measuring density
- From: Don Hutcheson <email@hidden>
- Date: Mon, 14 May 2001 17:46:28 -0400
Sorry if this post is a little late but I had to re-subscribe to the list
with my new e-mail.
1. Density is normally preceded or followed by the letter 'D' but I don't
know of any official convention. For example we use the term 'D-max' for
maximum density, but you can also write '4.00 D' or 'D 4.00'.
'Status T' density simply means CMY densities measured with the status T RGB
filter set. Different filtration sets have no effect on neutral density
(also known as 'visual' density) but will produce different values on
colored samples. The filtration status convention was an attempt to
standardize how densitometers measured color, but densitometer manufacturing
variables make the precision of any filtered readings questionable. Hence
you should only use a true spectrophotometer or spectrodensitometer if you
want accurate status-anything readings.
2. Density is the log to base 10 of opacity (O), where opacity is the
inverse of fractional transmittance (T) or reflectance (R).
The formula for density is:
D = log (1/opacity)
Example 1:
T (transmittance) = 25% (0.25 fraction)
O (opacity) = 1/0.25 = 4
D (density) = log (4) = 0.602
Example 2:
R (reflectance) = 50% (0.5 fraction)
O = 1/0.5 = 2
D = log (2) = 0.301
Every time you halve transmittance or reflectance you add approximately 0.30
density. This makes it easy to calculate approximate contrast, because every
time density range (DR) increases by 0.30 it doubles optical contrast.
Visual sensitivity is not exactly parallel to the density scale, hence at
higher densities the eye sees less change in apparent lightness or darkness
or contrast than the density scale would imply.
For example, we clearly see the change between middle-tone densities of 0.6
and 0.9 as an approximate 2:1 darkness increase, but if we increase solid
black ink density from 1.8 to 2.1 (the same optical increase) the eye barely
sees any increase in apparent ink density.
3. The formula to convert density back to transmittance or reflectance is:
T (or R) = 1/10^D = "One over 10 to the power of density"
This is useful for determining how much light actually gets through at a
certain density.
Example 3:
D = 4.0
O = 10^4.0 = 10000
T = 1/10000 = 0.01% transmittance
Hope this helps.
Don
*************************************
Don Hutcheson
Hutcheson Consulting
(Color Management Solutions)
www.hutchcolor.com
11 Turnburry Rd
Washington, NJ 07882
Phone: (908) 689 7403
Fax: (908) 689 5305
Mobile: (908) 500 0341
email@hidden
*************************************