PHYSICAL ASPECTS OF COLOURS IN LIGHT PAINTING
The first two parts of Colours in Light Painting article, was more of a theoretical nature. In this part, we are gonna deal with the technical implementation in light painting. Which colours are easy to use, which are not? Can I mix different colours in the Light Painting, and if so which and how?
In the Light Painting, you can use with the proper torch and the necessary experience almost any colour in any desired saturation and brightness. However, when trying to mix different colours, we are very often doomed to failure. Displaying “all” colours in an image like the “Photonenrotor” above is anything but easy. I worked on this picture for several days. The exposure time was 131 seconds. The blue light I achieved with a small torch with blue colour gel, both attached to a piece of acryl. On another piece of acryl, I stuck a second torch without gel. On this acrylic blade, I had attached outside two small pieces of colour gel (red and yellow). The third part was exclusively white (middle). By changing the focal length during the exposure, the traces of the blades are visible several times in this Light Painting. The outer ring with the colour gradient, I have realized with an torch with colour change mode. The difficulty was to adjust the rotation speed to the speed of the colour change and to determine the number of rotations for the desired effect. This soft light with the clean colour gradient can hardly be converted with a single rotation.
However, such light paintings only work without further light. By illuminating the room, I probably would have shone off half the outer ring again. It would also be impossible to make the outer ring much brighter. But why is it like that?
Back in school, we learned how to mix colours after subtractive colour mixing. From the 3 primary colours yellow, cyan and magenta all colours can be mixed. The more colours I apply to the sheet, the darker the mixed colour will be. If I paint all 3 primary colours in equal proportions on the sheet, the result is a dark brown, ideally black. However, in the physical sense, black is not a colour but the absence of light. Almost all printing processes use 4 colours: cyan, magenta, yellow and black.
The subtractive colour mixing, however, has no validity when I work with light. The more light I put on each other the brighter the result will be. In additive colour mixing, the 3 primary colours red, green and blue do not turn black but white. Neither can I mix brown with red and green light. I have no way to colour parts in the picture with light black, Black is in Light Painting always only parts of the picture without any light.
So in light painting, we first have to forget the knowledge of colour mixing that was learned at school and stored in our memory.
Theoretically, in light painting, you can render any colour as you would on an RGB monitor. Practically, this is very difficult. In the monitor, the individual pixels are cleanly separated from each other and do not affect each other. Every single pixel in the monitor is controlled with exact values for colour mixing and brightness. In light painting, we usually work with torches. These light up always a larger area, unlike the micrometre-sized pixel of the monitor. Also, there is no torch which has the brightness evenly spread across the light cone. Consequently, when using colour gels, the colour in the light cone is also not uniform.
In theory, the primary colours mix as follows:
RED – GREEN = YELLOW
GREEN – BLUE = CYAN
RED – BLUE = MAGENTA
RED – GREEN – BLUE = WHITE
In practice, this often looks different. I have a clean mix only if the brightness of the colours is identical, the colours are “clean” and the illuminated area is hit evenly by the light and the area reflects the light neutrally.
Here are some test shots on the white cellar wall:
As you can see, a clean mix of colours is not possible. I used two identical RGB torches here. The distance of both torches to the wall was the same. The torches do not project a clean, uniform cone of light. However, the secondary colours yellow, cyan and magenta are recognizable in some areas of the overlap. If I mount the torch on an acrylic blade or another light tool, which distributes the light quite evenly, it looks different. In that case, the light will not finally be reflected off the wall or any objects. If I do not know how to illuminate an object or part of a building with 2 colours, the result is even less clear.
Mixing the primary colours produces the 3 secondary colours yellow, cyan and magenta. What happens when mixing the primary colours with the secondary colours?
RED – CYAN = WHITE
GREEN – MAGENTA = WHITE
BLUE – YELLOW = WHITE
In large areas, the light is burned out here. Above the burned out spots it is easy to see how the colours mix. This is becoming more or less grey, and only in theory does this become white. If a larger area is lit up, it usually does not look good. Apart from that, I could then illuminate the scene with white light.
When working with several secondary colours or a primary colour and a secondary colour, you should always take care that you separate the colours in the illumination cleanly from each other to avoid these ugly grey areas. This is not so difficult in most rooms. For the illumination of people or objects, however, this can quickly become a major challenge.
When working with colour gels, nothing else happens when working with RGB torches as in the pictures above. The colour gel does not colour the light but only blocks all wavelengths except its own. A red filter allows only red light, the rest of the spectrum is blocked.
With colour gels, however, I can use other torches or flash units. With surface torches or a flash with a softbox, the light is softer and thus more evenly distributed. It does not burn out as fast as in my example pictures above. However, the area in which both colours illuminate the wall at the same time will be larger.
Above a practical example. Behind the enchanting Marla, I fired two flashes, one with a blue and the other with a green colour gel, several times. This created blue and green areas, areas in which the light burned out, that is white, is and which mix in which the two colours to cyan. In darker areas, the colour then goes somewhat towards grey, but this is mainly due to the reflection properties of the fog. After all, fog is not a clean reflector. But exactly this structured light with the many nuances I wanted to have here, a “clean” area behind the model would be very boring as I think.
Allways good light