How to Convert a 24-bit Image into Three 8-bit Images While Maintaining Brightness

When dealing with digital images, especially those in the realm of color management, the process of converting a 24-bit RGB image into three 8-bit greyscale images can be both fascinating and complex. In an RGB image, the three color channels (Red, Green, and Blue) carry the relative brightness information for each color component. This article will guide you through the process of separating these channels while ensuring that each individual image retains the same perceived brightness as the original composite image.

Understanding the RGB Model in 24-bit Depth

A 24-bit RGB image comprises three 8-bit channels, each representing the Red, Green, and Blue components. Each channel can hold values from 0 to 255, which is why 24-bit color can represent over 16 million colors. However, the brightness of each individual channel is relative to the maximum value it can achieve within this range. This relative brightness does not provide absolute brightness information, which means that simply copying the channels as greyscale images does not guarantee the same visual brightness as the original composite image.

Separating the RGB Channels in Photoshop

Using Adobe Photoshop or a similar image editing software, you can easily separate the color channels into greyscale images. To do this:

Open your 24-bit RGB image in Photoshop. Go to the Channels palette (Window > Channels). Here, you will find the color channels: Red, Green, and Blue. Right-click on any channel and select "Duplicate Channel". Save the duplicated channel as a new image file (File > Save As). Repeat the process for the other two channels.

These steps will give you three separate 8-bit greyscale images, each representing the relative contribution of Red, Green, and Blue in the original image.

Ensuring Brightness Preservation

While the above method provides the relative brightness information of each channel, it does not maintain the overall brightness level of the original image. To achieve this, you need to adjust the peak brightness of each individual 8-bit greyscale image to match the perceived brightness of the original composite image. Here’s how you can do it:

Open the 8-bit greyscale image corresponding to the Red channel. Go to Image > Adjustments > Brightness/Contrast. Adjust the Brightness slider to increase the brightness to match the perceived brightness of the original image. Save the adjusted image. Repeat the process for the Green and Blue channels.

It is important to note that adjusting the brightness does not change the absolute values in the image data. Instead, it scales the values to ensure that the greyscale image appears as bright as the original composite image.

Understanding the Display Luminance Contribution

The relative brightness of the Red, Green, and Blue channels is influenced by the display’s white balance settings. Generally, the display white balance determines the contribution of each primary color to the overall brightness of a displayed full-color image. The distribution typically follows this pattern:

Green accounts for around 60% of the "white" luminance. Red makes up approximately 30%. Blue contributes only about 10%.

Even though the image data remains unchanged, the perceived brightness of each channel is adjusted by the display based on its white balance settings. This means that while the greyscale images will maintain the relative brightness levels, the perceived brightness in relation to the display settings will also be adjusted.

By understanding these principles, you can effectively separate a 24-bit image into three 8-bit images while preserving the brightness. This technique is particularly useful in scenarios where you need to process individual color channels separately, such as in color correction, image analysis, and various post-processing tasks.