Lab Color - Grayscale Channels

[michi8]

Hi, I am seeking advice on Lab Color mode. I'm to testing if differences can be visually seen when comparing different blue pen inks. From what I see when I split the channels into grayscale is that the blue/yellow 'b' channel shows differences in inks that are a medium-bright blue compared to a dark/indigo blue. The brighter blues seem to be darker in the 'gray' shade than the indigo/purple in the grayscale 'b' channel. But I have found that when I compare writing made by pens with medium tip to those with fine tip, then regardless of colour the thicker written ink line appears dark in the 'b' channel. I thought the grayscale channel converts the separated colours into gray shades only. Does it also pick up levels of darkness and brightness even when the 'L' channel is not used? 

[John T Smith]

Please post the exact name of the Adobe program you use so a Moderator may move this message to that forum

[michi8]

Thanks John,

I am using Adobe Photoshop version 22.2.1

[c.pfaffenbichler]

Could you please post screenshots taken at View > 100% with the pertinent Panels (Toolbar, Brush Settings, Layers, Channels, Options Bar, …) visible? 

[c.pfaffenbichler]

What are you trying to achieve anyway? 

You could get Lab-readings when working in an RGB image, so are you deliberately working in Lab? 

If so, do you work in 16bit? 

[michi8]

Thanks for replying 🙂 

I am working in 8bit mode and in the Lab Color mode or colour space. I am deliberately working in Lab Color mode to test a method that previous studies have shown that can help detect different black pen inks. But I am testing it for blue pen inks. The method is to open an image of two different coloured blue inks in Photoshop and follow these steps:

Select Lab Color mode > hide channel L and 'a' channel > split image (changes to grayscale) > apply auto-levels contrast to a and b channels for enhancement 

Then after contrast applied see if there is a difference between the inks. In 'a' channel it's just bright white but in 'a' channel some inks look dark grey next to a black ink but other images of two different inks just look black. Could the contrast be making it difficult to tell ?

[D Fosse]

The a and b channels just map saturation along the two axes. But the darker the ink is, the lower the saturation is likely to be. When you get to black, there's no saturation possible at all.

A normal "blue" ink will probably have a magenta component as well. You can use Image > Calculations to combine the two channels and extract the maximum information from both.

[michi8]

Hi D Fosse, please see my reply above to c.pfaffenbichler. 

It seems your answer on saturation is what I am seeing in the a and b channels. Some blue inks I am testing do have magenta in them from what I can see in the composite image of a and b channels (which is displayed in colour) but the method I am using to test different blue inks isn't that successful it seems and I am trying to see what is possibly causing the processed image (after contrast etc) to just look black or white while some inks show differences. 

[michi8]

Sorry, so if I understand right, the grayscale a and b channel images are showing the separated colour components in those channels but in grayscale including the saturation of colour along the two axis of red-green values and blue-yellow values? 

[c.pfaffenbichler]

Do the images contain anything except the substrate and the pen strokes? 

Could you please post screenshots?