LRGB TECHNIQUE - INTRODUCTION

The LRGB technique involves taking individual images in red, green and blue. These images representing the primary colors will then be stitched together to produce a color image. To achieve this type of image, we use a monochrome CCD camera (black and white) capable of reproducing the color spectrum if we add to it individually, during the shooting, filters representing the primary colors red, green and blue. . The term LRGB represents the following light spectrum of the acquired image:

  • L represents the Luminance of the object
  • RGB represents primary colors Rouge, Verte and Bleue to reproduce the entire color spectrum of the object

Without going into too much detail, Luminance images serve to add depth to the resolution of the image. For example, a 10 minute exposure of Luminance (without a color filter) will reveal more detail (depth) of the object than a 10 minute exposure with a Red, Green, or Blue filter. The Luminance image will be monochrome, but when it is overlaid on the RGB image, it will be in color. When making the images, luminance images are given a longer exposure time than RGB images. Also, RGB images can be taken in Bin 2 × 2 to acquire more light and thus reduce the exposure time. In addition, consider that the RGB color image has a lower Signal / Noise (S / N) level than the Luminance image. This means that the difference between the signal (the resolution of the image) and the background noise is lower, thus limiting the processing to resolve the image. As the Luminance image has a higher S / N level, it benefits from a greater difference between image resolution and background noise. Luminance image processing will therefore provide more detail and depth in image resolution than the RGB image.

To take each image, we add a filter wheel in front of the CCD camera. We put a clear filter for the luminance images to ensure that these images will be taken in parfocal (same focus). Here is an example of a CCD camera with its motorized filter wheel:

To automate the shooting, the filter wheel is actuated by a motor which rotates it, allowing it to pass from one filter to another without the intervention of the operator. The latter only has to determine in advance the exposure time for each filter. The filter wheel can accept the addition of filters other than the standard LRGB filters. For example, the H-Alpha (Hydrogen-Alpha) filter can be used as a luminance image to bring out the light spectrum of emission nebulae and darken the sky (HaRGB). Several other filter combinations can be used. For example, see the section of the site onUse of narrow band filters to combat light pollution.

There are color CCD cameras on the market that capture all colors in the same exposure. So, why use a monochrome CCD camera with LRGB filters? The answer to this question is that the LRGB image will have a more precise resolution and provide more depth to the image than the color CCD camera. Indeed, the color camera uses one pixel for each color (four pixels are used to reproduce red, green and blue) while the monochrome camera uses the same pixel for the 3 colors which produces an image of better resolution. In addition, the luminance image will better resolve the image in depth. For the same light sensitivity of the camera sensor and for the same exposure time, a luminance image acquires up to three times more light than an image produced in RGB (Bin 1 × 1). Also, a monochrome camera can be used for imaging with filters other than standard RGB, which is a major advantage.

For the preprocessing of LRGB images, the same techniques are used as those described in this site. All images are taken in black and white FIT format. We then proceed to the individual compositing (assembly) for each of these images. The RGB images are then stitched together in Photoshop to produce a color image. The RGB image and the Luminance image will be individually processed using the techniques of image processing of this site. Subsequently, the luminance image will be superimposed on the RGB image to add depth to the resulting image.

As we have just seen, making an LRGB image requires individual exposures for each layer of luminance, red, green, and blue. It will therefore be necessary to plan for a long exposure time. Some amateurs using this technique expose for a whole night and even more.

For those who have a color CCD camera or a digital camera (APN), it is possible to create a Synthetic Luminance image from the RGB color image thus making the LRGB technique accessible to users of these devices. See section Create a synthetic luminance image for all the details on creating a Synthetic Luminance image and stitching it to the RGB color image. 

Richard Beauregard
Sky Astro - CCD

Revised 2021/03/24