Dimmable studio light with colour mixing

Date of publication

by Felix Jakob Fliegner

As part of his electrical engineering studies at TU Berlin, Felix was our intern in early 2017. This is his report on his student research project, which is well worth reading.


This project is about the conceptual design of an LED panel to illuminate a scene in a studio for portrait photography or video shooting, where lighting conditions similar to daylight are required. This report covers the entire project period from initial theoretical considerations to production and commissioning in a sketchy manner and is intended to enable replication or simply provide some inspiration and background knowledge. For this purpose, the basics of lighting technology relevant to the project are first summarised and then applied to the project implementation.

Essence of light

Light describes the part of electromagnetic radiation that causes a sensation of brightness in the human eye. The terms light and colour are therefore inseparably linked to the biological constitution of the human eye and always presuppose a sensory perception of the radiation by the cones in the eye. For the construction of a lamp, therefore, appropriate illuminants must be used that are oriented to the spectral brightness sensitivity of the human eye (V(λ)-function). The decisive factor for the illuminant is therefore not only the radiant power itself, but its spectral weighting with the human luminous sensitivity. This results in the luminous flux, which, measured in lumens [lm], describes the total visible radiant power emitted into the room. It is a good measure for the rough classification of luminaires.

For studio lighting, solutions are of particular interest here that not only have a high luminous flux but also a high illuminance, measured in lux [lm/m²], i.e. provide as much luminous flux as possible per receiver surface (here: a face or an object). This requires focusing the luminous flux on the corresponding solid angle, which is described by the third photometric quantity candela [lm/sr]. If there is still a lot of light coming from the smallest possible solid angle (expressed in steradians, sr), then the luminaire is highly focused. A large part of the luminous flux therefore actually reaches "its" target object. Such behaviour is useful for theatrical spots or torches. In the same way, luminaires with less strongly focused light (e.g. for room lighting) are of course also common.

Für die Anwendung in diesem Projekt wird auf die Fokussierung nicht zu viel Wert gelegt, da sie die Konstruktion schnell zu aufwendig machen würde. In diesem Fall wird eine hinreichend große Lichtfläche konstruiert, die in der Anwendung nicht zu weit vom Objekt entfernt stehen soll und somit die gewünschte Beleuchtungsstärke auch ohne Fokussierung erreicht.

Objective of the project

The aim of the project is to construct a daylight-bright studio luminaire that can reproduce different colour temperatures of daylight indoors. Of course, it is not a matter of the same radiant power as that of daylight, but only of a simulated brightness perception. The colour spectrum must therefore adapt over the course of the day. Another goal should be to provide the "illuminated studio" service as inexpensively as possible and with little construction effort.

Daylight changes its colour composition in the spectrum during the course of the day. In the twilight hours immediately after sunrise or before sunset, the proportion of short-wave (red) light is particularly high because the sun is very flat and the rays have a correspondingly longer path through the earth's atmosphere than at midday. In the atmosphere, long-wave light is more strongly refracted, bent or even reflected. UV radiation and long (blue) light waves therefore reach the Earth's surface in smaller numbers the further their path is from entering the atmosphere to the surface. Short-wave (red) light or even infrared radiation, on the other hand, reach the surface almost unhindered. The same applies to the composition of daylight at midday. Measured in absolute amounts, the red component is higher than at dusk. However, the blue component is also higher and, when mixed, leads to brighter and bluer light. Photographing at midday is therefore not particularly attractive, as the colours appear quite cold and faces in particular can look sallow. It becomes more attractive in the so-called "golden" hour shortly before sunset, when the colours become "warm" due to the effect described above.

All in all, daylight is perceived by people as "white" throughout the course of the day. However, every incorrectly calibrated camera with a faulty white balance proves how great the differences can be. The sensor does not have the brightness sensitivity of the human eye mentioned at the beginning. It measures the irradiance per colour and pixel and would produce reddish images at dusk and bluish images at midday. A control mechanism (the white balance) must therefore have information available for each image as to which mixing ratio is perceived as "white" by the human eye at a given time of day. Illuminants must therefore also enable this shift from "warm white" to "cool white" and back again.

For the studio luminaire, this obviously means colour control (dimming) of the blue component in the emitted luminous flux. RBG LEDs that enable separate control of the light colours red, blue and green would be conceivable. On the one hand, however, these are very expensive if they are to emit the required high luminous flux, and on the other hand they make manual colour mixing quite complicated. For an initial prototype, therefore, a combination of cool white and warm white LEDs is more likely to be considered. They are much cheaper and completely sufficient for this purpose.

Materials for the inexpensive construction of the lamp

LEDs are now available in all shapes and colours. LED light strips are suitable for maximum design flexibility. They come in the desired white gradations of cold, neutral and warm and are also easy to cut to size. This considerably reduces the amount of soldering and gluing required. Another advantage of the light strips is their low heat emission. With this construction and operation, it is therefore not necessary to pay attention to cooling (Attention: this does not apply to LED strips in general, but this special wiring as explained below! Overheated luminaires pose a serious danger to people and the environment).

For this project, a combination of neutral and cool white LEDs is used. The reason for this is an almost true-colour reproduction of midday light when both strips shine equally brightly (see below). The two types of strips should be arranged alternately on the panel in equal numbers, so that a mixed colour of both lights is produced on the illuminated object. The geometry of the stripes results in an efficient design with an almost square base that is completely covered with alternating luminous stripes arranged next to each other.

Any type of solid building material (wood, metal, plastic) would be suitable as a substrate. In this case, artificial leather mats are used. They are not only inexpensive and very easy to cut to size, but also offer maximum flexibility in handling later for any desired bending of the panel. The artificial leather surface is stabilised on the one hand by the conductive wires and on the other hand by a ring wire (from garden fence supplies), which is laid around the entire arrangement of the LED strips.

Common stranded wires with a cross-section of 0.5 mm² are used for the power supply. Less would also be sufficient, but in this thickness the wires can still contribute to the overall stability of the panel. The supply line of the panel is a commercially available mains cable with associated power supply unit (see component list).

Since dimming of the luminaire is necessary, a mechanism must be found that can regulate the LED efficiently. The classic principle of the potentiometer, as is usual for light bulbs, cannot be used here. LEDs are semiconductors and behave differently in the circuit than ohmic resistors or light bulbs. Basically, each type of LED draws a specific current that must not vary over the duration of operation. In addition, a certain operating voltage is necessary, which is variable within certain limits but does not seem suitable for dimming. The LED strips already regulate the current internally, only the total power of the power supply unit must be designed for the strip and provide a correct voltage. To influence the brightness of the strips, so-called PWM dimmers (pulse width modulation) can be used. They divide the current flow without changing its amplitude. This means that a constant direct current no longer flows per second, but a direct current that is interrupted every hundredth of a second, for example. The LED is therefore switched on and off in a very short time interval. This process is not visible to the human eye, yet it becomes noticeable as soon as the interruption periods become longer and longer (short on, long off). This leads to an overall darker perception of brightness in the eye and allows the LED to be dimmed, which in itself does not shine any darker, just less frequently or more frequently per second. Such a dimmer is needed for each LED strip so that the intensity of cold and neutral white light can be controlled independently on the rotary controllers, thus allowing the light of the entire panel to be mixed.

The following list of components results from the preceding considerations:

  • 1x LED strip neutral white, 5 metres, 12 V
  • 1x LED strip cold white, 5 metres, 12 V
  • Imitation leather from the running metre (1 m x 0.5 m)
  • Wire strands 2-3 m bicoloured 0.5 mm² cross-sectional area
  • Fence wire for stabilisation
  • PWM dimmer
  • Power supply 18 V, 1000 mA (commercially available laptop power supply or similar)
  • Textile glue, all-purpose glue and silicone if necessary
  • Solder and flux
  • Masking tape as a fixing aid for everything that requires a third hand
  • General tools for electrotechnical work

Construction of the lamp

Cutting and fixing

First, cut the LED strips supplied into 30 cm sections and do not mix the resulting piles for neutral and cold white (if this happens, the strips can usually be distinguished by their slightly different LEDs; if necessary, test them on a laboratory power supply). The artificial leather should also be cut to size so that there is an almost square base for the strips that will later be arranged next to each other. (It is always possible to cut the strips where they are marked. This may result in 35 cm or 20 cm long segments). Suitable dimensions for this project are therefore e.g. 38x40 cm. It is important not to make the overhanging edge too small, as it will later have to accommodate all the cables and stabilising wires.

In the next step, all the strips are glued to the rough side of the imitation leather. It is advisable to use some textile glue in addition to the self-adhesive property of the strips to ensure lasting stability. In this step, it is important to alternate the strip types (neutral and cold) and, for the aesthetically ambitious, to always use the same orientation of the strips so that all (non-symmetrically) arranged series resistors are later in the same place. Electrotechnically, there is nothing to consider in this step. The masking tape serves as a fixing aid for curling strips. The glued strips should rest under load for at least 12 hours so that the textile glue can harden. It makes sense to weight them down with books. Some paper should be placed on the LEDs as a protective layer.

Wiring the wires

As soon as the glue joints have cured satisfactorily, you can start wiring the strips. To do this, always connect eight strips of the same light colour in series at their plus and minus poles. So first count the eight strips from one side and then the other eight from the other side. Finally, there should be four loose wire ends in the middle (two minus and two plus poles). These are now wired parallel to each other rather than in series, so connect the positive pole of one block of eight to the negative pole of the other. The remaining positive and negative poles form the connecting wires for the supply line from/to the dimmer.

A similar connection is made for the other colour of the strip. For reasons of space optimisation, it makes sense to wire one light colour on one side and the other light colour on the other. Technically, it does not matter where the current for a strip comes from as long as the polarity at the marked connections is observed.

The final four connecting wires for the two dimmers can be led along the edge of the panel to a common point and out to the back (through the base). The dimmers are now attached here on the back.

Each dimmer is characterised by two inputs (positive pole and negative pole) and two outputs. The inputs of both dimmers are connected in parallel, so they get the same voltage from the power supply at the end. As these PWM dimmers have an internal current control, the parallel connection of the dimmers and thus of the neutral or cool white LED strips is not critical. Connect the positive and negative poles of one group of light strips to one dimmer output and the two wires of the other group to the other dimmer output. It is important to note the polarity, so it is advisable to use different coloured wires.

For the mains supply, a simple laptop power supply unit is used in this project, as these are very easy to obtain or are even available second-hand and save money. The usual output voltage of these power supplies is 18 - 20 V. This is too much for an LED strip, which is usually designed for 12 V. Therefore, the circuit in this project uses a simple laptop power supply. Therefore, the wiring in this project uses the trick of connecting two LED groups per light colour (the 8 strips mentioned above) in series. By connecting them in series, the entire voltage in this branch (18 V) is divided into two groups of 9 V each. The operation of the light strips is therefore below the usual design. As already described, a slight variance in voltage is not critical for LEDs. At 9 V, they simply do not shine as brightly as they theoretically could. In total, however, their luminous flux is still sufficient and at this voltage has the positive side effect that heat emissions are considerably reduced. For studio use, this argument is quite relevant and also ensures that the artificial leather with the textile glue is not overheated.

Closing

Now that all the electrotechnical components have been completed, it is time for the final stabilisation. For this, normal steel wire from the garden fence is used and bent so that a frame is created once around the panel. This frame is placed on the leather surface and the protruding edges are folded over on all four sides. A mixed application of silicone and all-purpose glue is suitable for gluing. The latter provides a quick fix until the silicone has cured and ensures long-term stability. Silicone is also flexible enough to bend the whole assembly if necessary, in case curved light sources are needed. 12 hours should also be sufficient for this bonding to dry.

The PWM dimmers on the back of the panel can also be glued on, protected by leather scraps. If you want to increase flexibility, do not connect the mains cable directly to the inputs of the dimmers, but install another socket in the arrangement that corresponds to a plug on the mains side. This way, the cable and the power supply can be stored separately from the panel for transport purposes.

After completing these steps, a functional panel should be available. It can be refined by electronically controlling the dimmers so that the light can be controlled via a computer. In addition, other support surfaces (wood or similar) are conceivable, which seem more attractive for certain applications than the rather unstable-looking artificial leather construction. On the other hand, it is precisely this flexible arrangement that offers a very wide range of uses for which otherwise new lamps would have to be procured in each case. The overall size of the panel can also be varied. The power supply may change, but the basic principle remains the same. The basic version presented here therefore offers a good starting point for your own further developments and improvements.

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