How do I map grayscale values to get temperature?

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PRAVEEN KUMAR 2022-5-20

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https://ww2.mathworks.cn/matlabcentral/answers/1723490-how-do-i-map-grayscale-values-to-get-temperature

评论： Walter Roberson 2022-5-22

Thermal_Image.jpg

In the attached file I have shown the thermal image clicked during live preview from my camera.

I am able to fetch the Minimum, Maximum, and Mean Grayscale Level values along with GL value at Center (Blue Circle) at backend continuously in live video.

I want to know if I can use these GL values to get corresponding temperature with any equation or any function.

Also can you suggest anything else that I can use for this purpose?

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PRAVEEN KUMAR 2022-5-20

@Image Analyst

@Walter Roberson

I would be very thankful if you guys can provide some insight and guide me.

Image Analyst 2022-5-22

@PRAVEEN KUMAR Look to my answer and comments below.

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Answer 1

Walter Roberson 2022-5-20

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https://ww2.mathworks.cn/matlabcentral/answers/1723490-how-do-i-map-grayscale-values-to-get-temperature#answer_968140

"I want to know if i can use these GL values to get corresponding temperature with any equation or any function."

No, there is way to do that based on the grayscale level. That colormap is not linear in grayscale brightness.

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PRAVEEN KUMAR 2022-5-22

Thank you for your response @Walter Roberson & @Image Analyst

Here is the piece of code i am running.

clear cam;
cam= webcam('PureThermal (fw:v1.3.0)');
preview(cam);
global take_photo;
take_photo=1;
while(take_photo == 1)
    data = snapshot(cam);
    gray=rgb2gray(data);
    minGL=min(gray(:));
    maxGL=max(gray(:));
    meanGL=mean2(gray(:));
    minGLValue=int2str(minGL);
    maxGLValue=int2str(maxGL);
    meanGLValue=int2str(meanGL);
    
    
    app.MinGLEditField.Value= minGLValue;
    app.MaxGLEditField.Value= maxGLValue;
    app.MeanEditField.Value=meanGLValue; 
    
    x= str2double(minGLValue);
    y= str2double(maxGLValue);
    c=x*2.083;
    d=y*0.1286;
    a=app.ToneImageSlider.Value;
    bw=gray>a;
    
    imshow(data)
    %imshow(data,'parent',app.UIAxes);
    title('Structural Fault Detection',"FontSize",14);
    colorbar;  
    caxis([x y]);
    axis on;
    colormap("jet");
    [L n]=bwlabel(bw);
    
    stats = regionprops(L, 'BoundingBox','Centroid');
    row=80;
    column=60;
    center=[row column];
    Inttensity=gray(column,row);
    intt=int2str(Inttensity);
    app.CenterEditField.Value= intt;
    viscircles(center,5,'color','b');
    
    
    
    hold on
    
    for object = 1:length(stats)
        bb = stats(object).BoundingBox;
        bc = stats(object).Centroid;
        rectangle('Position',bb,'EdgeColor','r','Linewidth',2)
        plot(bc(1),bc(2), '-m+');
        
        if length(stats) <=5
            
            a=text(bc(1)+15,bc(2),strcat('X: ', num2str(round(bc(1))),'    Y: ',num2str(round(bc(2)))));
            
        end
        
    end
    
    hold off
end

Please see and suggest me what to do next.

Walter Roberson 2022-5-22

PureThermal is a FLIR camera.

There is no possibility that converting a color FLIR image to grayscale will work properly for your purposes. FLIR does not use a linear colormap.

If I understand correctly, PureThermal (without the 2 of PureThermal2) is a board with a sensor and a FLIR Lepton socket. In particular, as far as I can see, it is not one of FLIR's cameras that has WiFi.

If my understanding is correct, then you have three choices:

calibrate your equipment in order to determine the color to temperature mapping. I am not confident this will work: based on my past reading about FLIR, the lower-end devices appear to automatically determine temperature range to use. However, you might succeed in determining the colormap, and then for any given image if you have two calibrated spots you could determine the rest of the temperatures; OR
You can purchase the FLIR research software;
You might be able to take the sensor and Lepton module and put it into a development board that provides better software

If using the FLIR research software is not an option, then adding another development board is probably not an option; then your only option might be calibration overall, together with having two points of known temperature in each image.

FLIR sells at different price-points, and it is deliberate that their less-expensive equipment makes it difficult to get actual temperature data out.

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