Jump to the step by step instructions ↓

EDN Color Blocker is a free program that, in a few milliseconds, automatically calculates the maximum and optimal blocking color. This color is then used to make so-called transparent digital negatives. We will soon discover that black negatives are not always the best choice for making negatives.

Some theory

The first photographers discovered almost 200 years ago that UV light is best blocked by red and least by blue color. And this knowledge has been used in retouching the first paper negatives, to make safe red lights in the darkrooms, etc. Nowadays, however, these discoveries have become relevant again by printing on transparent foils. 

Printing negatives using different colors has several advantages over printing a negative with black color. Two advantages are essential to us. In some cases, different colors block UV light much more strongly than black, and in other cases, specific colors significantly increase the range of tones that can be used to make corrections. I named the color that blocks UV light the most the maximum, and the color that blocks UV light the most evenly I called the optimum UV light blocker.

Maximum blocker

We use the maximum blocker in case we want to achieve the maximum black and maximum white color in the photo. Such an example is found in photographic techniques with an extensive tonal range, such as in platinotype or salt printing. 

We often want to achieve extraordinary blacks here, but at the same time, we don’t want to lose the bright tones. In the fictional example below, we printed the negative using black ink using a cheap printer (1). Because we exposed the photo for too long, we lost the bright tones in the photo (2). 

If we want to show white tones, in this case, using black negative, we need to shorten the exposure time. But by shortening the exposure time, all the tones in the photo are also brightened. The image becomes too bright (3). 

We soon find that the solution is in red, which blocks light much better than the black color. In the next test, we exposed the photograph with the same time as in the previous case, but this time we printed the negative in red color (4). 

We notice that the red color blocks UV light much better. We notice much more clean, light tones in the photo (5). 

Red color, therefore, blocks UV light better than black. Now we could even further increase the exposure time, which would most likely create much darker tones in the photo. 

The color that most strongly blocks UV light in a particular photographic technique is called the maximum blocker. 

Optimum blocker 

Quite a few users are surprised to discover that even cheap printers can block UV light very well. Today, we are almost no longer looking for the color that blocks UV light the most, but we are looking for the color that blocks UV light the best. And this is also another advantage of colored negatives. 

When using black ink to print negatives, we notice that we sometimes have a fairly small number of color tones available in a photo to make corrections, so it is almost impossible to make “perfect” corrections. 

In the cyanotype example below, we observe that in a sample with 101 color fields, only about 23 tones with different values are available to calculate corrections (6). 

To remedy this shortcoming, I started looking for new solutions years ago. First, I replaced the old 101-field step wedge tables used by photographers until then with a 256-field table. Users of EDN negatives now have as many as 62 useful shades available when using the table with 256 squares (7). 

But even a table with 256 color shades has proven useless when printing with photographic techniques with an even more limited color range. One such example is the oil print photographic technique shown below, with about 35 usable tones (8). 

We needed a new solution, and this one was in the use of different colors. Namely, different colors not only create different maximum UV blockers but can also display different color ranges. Returning to the previous example, we found that we can show, using a black negative, 23 usable tones in a table with 101 fields (9). But using a reddish negative (HSB 330-100-0), we can display as many as 57 tones (10) in the same step wedge table with 101 fields. The resulting curve is almost closer to the ideal, the linear line on the graph. A negative printed with the selected reddish color show an almost perfect dynamic range of tones, unlike a negative printed with black. The photo needs almost no more corrections. And to this color for negative printing, which most increases the number of usable tones, while still retaining whiteness in the photo, I gave the name optimal color blocker.

About EDN ColorBlocker 

Older versions of the EDN ColorBlocker program were scripting programs that could only be run in Adobe Photoshop. However, because the ColorBlocker version 3.0 is a web program, it now works on all operating systems. It runs on almost all web browsers such as Safari, Microsoft Edge, Firefox, Opera, Google Chrome, etc. 

Another great advantage of the new version of the program is that the maximum and optimum blocker can be used in Adobe Photoshop and in GIMP, Affinity photo, Adobe Lightroom, or in any program that supports LUT files of type cube. 

The third novelty in this version is a new way of using the calculated color value. If, in the previous version, we used the blending mode Screen and the Colorize command found in Adobe Photoshop, in this version, the results are displayed via LUT files or gradient map files. These are some kind of programmable LUT table. In these LUT and gradient files, we can specify the intermediate color tones, which can be used to specify the color distribution in a photo. 

In cyanotype, using a black-and-white negative, we were able to display 23 useful tones on the negative (11). Using a color negative and coloring the negative with Screen mode (12), we were able to show 35 tones. With Colorize command, we were able to show up to 50 tones (13).

And with our latest technique, we have increased the number of usable tones even slightly. But more importantly, the tones are now displayed in an almost ideal, linearized form (17). To create a “perfect” photo, we need only a small correction using the EDN Digital negatives program.

How does the program work?

In previous versions of EDN ColorBlocker, we had to color the negative with the selected color. As mentioned, we were able to use the Screen or Colorize commands. 

In the newer version of EDN ColorBlocker, when the program process the desired color, it first search for the darkest and then the lightest shade in the corresponding HSB color (14). 

Then, to better balance the curve, the program searches a color value that determines the middle value inside the color range (15). 

The number of usable tones is now increased (16), even in photographic techniques with a relatively small tone range. 

But, because of the introduction of a middle tone in the blocker, we notice another great advantage. The correction curve is almost completely flattened (17). 

And that’s not the end. EDN ColorBlocker has another big advantage over the “manual” process I described in Easy Digital Negatives. 

Due to the technology, which is much more accurate than our human perception, the program now considers quite a few additional criteria when searching for the maximum and optimal blocker. 

When searching for color, EDN ColorBlocker first looks for some of the colors that best block UV light. It then eliminates all color columns that create “posterization” or better-said inversion of tones. 

In this step, the program marks the color that best blocks UV light. That becomes the color of the maximum blocker. 

In the next step, the program finds the columns with the largest number of color shades. From these columns, choose the one in which the shades are furthest from each other. 

In the last stage, the program chooses the color columns in which the so-called standard deviation from the ideal values is the smallest. 

In short, the program finds the color that best linearizes the shades in the selected photographic technique. 

Instructions of making a colored negative

Making corrections for transparent digital negatives consists of a few steps. This chapter will learn how to increase the range of negatives with a limited tonal range.  


First, we download from the Internet the HSB palette file. 

The HSB_Grid_v2.tif file is NOT mirrored or inverted. We simply print it on transparent film. 

To make a sample print, we need to know the standard exposure time, but we can also use the approximate time. We help ourselves with the printed HSB table. 

When the image is printed, both light and dark tones should be visible. 

The sample is digitized, scanned, or photographed. 

If we photographed the sample, we need to assign the right size and resolution to the digital image. 

We open the digitized image in the EDN ColorBlocker program, which instantly calculates the most suitable color for transparent negative.

We save the file with corrections in the appropriate folder. 

We open the negative color file, which will change the color of the negative.


First, we add corrections to Easy Digital Negatives, then we invert it, mirror it, and finally add the negative color just calculated. 

We can now print the colored negative. 

Making a color negative

Because EDN ColorBlocker version 3.0. program is a web application, it works in all operating systems, and due to the use of LUT files, we can use it in all latest photo editing programs. 

We can run the program via the website or downloaded it at our home computer. 

We will only need the HSB_Grid_v2.tif file (1) and the ColorBlocker program to check the blocking color. Since working with this program is the same as working with the EDN program, we will describe the steps a little more briefly.

1. Downloading a sample HSB file  

2. In the Downloads menu, we find the file EDN_HSB2.tif (1). 

3. We right-click on the file name and download the file to our home computer.  

2. Printing an HSB file

1. We open the HSB_Grid_v2.tif file in the photo editing program. 

2. Because the file is ready to print, it does not need to be mirrored. Images should also not be changed to Grayscale mode. 

3. We load the transparent film into the printer and select file> Print command. 

4. If we have the option, we select the Printer Manages Colors option

5. In the advanced printer settings, we select the best possible print quality (2), and we turn off all automatic printing modes or select manual settings (3).

6. The photo of the sample should be printed in color mode. 

7. Once the photo is printed, we wait about an hour for the ink on the transparent film to dry completely. 

8. If possible, we save the printer settings, as we will now use them to print corrections. These print settings are the same as the settings for printing step wedge files from Easy Digital Negatives.

The printing of this color table is quite simple, but I have to re-remind you that all of the parameters that affect an image’s quality must always be the same. Otherwise, we will get unreliable results.

3. Exposure time

Even when creating a table to discover the best-blocking color, the exposure time is essential. Similar to the description of the EDN program, the following rule applies. 

• If both white and dark tones are visible in the HSB color palette, the HSB table sample is exposed correctly (4). 

• If light tones are not visible at the top of the image, the HSB table is too bright (5). In this case, we shorten the exposure time by half and reprint the table. 

• If only light tones are visible in the table, the exposure time is too short (6). In this case, we need to print a new HSB table on paper. We now double the exposure time.    

4. Making a positive image

The production of a positive image depends on the photographic technique we use. In most cases, however, the transparent negative is turned towards the paper with the printed side (7).  The paper and the negative are squeezed together and exposed for the selected printing time.

5. Scanning samples

Once the sample is printed and dried (8), it is time to scan or photograph it. 

1. We place the image of the sample in the scanner, but before scanning, we adjust some settings (8). 

2. First, we select color scanning with 24-bit depth or millions of colors. 

3. If there is an option, we select the best scanning quality. 

4. The scanning resolution of our samples is 300 dpi. 

5. Then we choose the Color scanning or sRGB color space. 

6. In the next step, we click on the Overview or Preview button and select the scan area in the photo. This area is limited only to our sample. It is best to enter the exact value (2895 x 1711 px). We position the scanning surface as precisely as possible in the corners of our sample. 

7. We scan the sample by clicking on the Scan button.

6.  Preparation of a sample for reading data

If the sample has not been previously adapted to work with the EDN program, we must change the sample to the appropriate size in the digital photo processing program and assign it a resolution of 300DPI. 

In the links below, you can access quite a few procedures to resize an image.  

Cropping EDN samples in Photoshop, Cropping EDN samples in Affinity Photo and Cropping EDN samples in GIMP

7. Running EDN ColorBlocker program

Finding the right color is extremely easy. 

1. If we are using the EDN ColorBlocker web program, we open any web browser, and we go to the 

2. In the Run menu, we find the EDN ColorBlocker v.3.0 command. 

3. The scanned HSB file can be opened in the program by clicking on the Choose Files button, but it is even simpler to select the file with the mouse and drag it to the Choose File button (9).  

8. Saving corrections

When we open or drag and drop the HSB positive image file into the Choose File button, ColorBlocker calculates the so-called Optimum and Maximum blocker. In most cases, we use optimal blockers, enabling us to make as perfect as negative as possible. Color gradients with blocking colors are stored in different file types (LUT and gradient map). In this way, as many users as possible can use them. 

In our example, we will download the so-called Lookup Table by clicking on the Optimum LUT 1D (10) link. These files can be used in almost all photo editing programs.

1. We save the chosen blocking color by right-clicking on the desired link and selecting the command Save link as .., Or Download Linked File As and the like. 

2. In our case, we click the Optimum LUT 1D link (10). 

3. In case of increased computer protection, a warning appears asking us if we want to download the file. Of course, if we want to transfer data, we must allow the transfer.

9. Changing the color of the negative

1. We open the photo we want to convert to negative and change its color. 

2. We process the image according to the previous chapter’s description and add a correction to it at the end of the processing. We add a curve (11), which was created with the Easy Digital Negatives program. 

3. The image is now inverted into a negative and mirrored horizontally (12). 

4. All layers must be now flattened with the Merge Layers or Flatten image command. 

5. Now we can change the color of the negative. We will change the color via a previously saved LUT file. 

6. We click in the Adjustments tab (13). 

7. Then we click on the button Create New Color Lookup to insert the LUT files (14).

8. The Properties tab (15) opens, in which we click on the Load 3D LUT option (16).

9. We can now locate our file with the Optimum LUT 1D file and upload it to the program. 

10. Our photograph is suddenly colored with cyanotype hue (17). 

11. When the image is changed to a colored negative, we print it using the same settings as we used to print the HSB sample. 

The workflow is also shown in the videos on the Videos/ColorBlocker page.