By Fabio Sorabella
Introduction
The purpose of this article is to explain some general concepts about the problems related to the use of multi-colour sources in LED projectors for events and film productions, which are related to the physical features of the chips and some external factors, which by nature determine their inconsistency.
PROLIGHTS has developed a series of techniques, production processes, quality control, and software technologies that allow it not only to control and limit these phenomena by offering consistent products, but also to offer increasingly advanced functionality to its users. This ecosystem is branded under the SPEKTRA label and we will see some of the advantages of this below.
The origin of the inconsistency problem in multi-colour projectors
The problem of inconsistency is inherent in the very nature of LED chips. LED chips are made of materials that are not always consistent, which is why even if one chooses good quality components from qualified chip manufacturers who offer careful chip selection (binning), these are always expressed in range values, and never absolute, and normally involve a combination of different tolerance ranges for each of the following specifications:
– Spectrum Tolerance: dominant colour wavelength expressed in nm
– Luminous flux level: expressed in lm (lumens) or mW (radiometric power)
– Tolerance on voltage drop: expressed in Vf
The combination of all these tolerances creates a large margin of error when trying to achieve consistent colour reproduction performance between multiple LED projectors.
The greater the number of colours used within the projector, the greater the diversity and complexity in predicting expected values from the combination of these ranges and which will increase exponentially as the number of colours in the projector increases (4 colours, 6 colours or more).
In order to understand this, below is an image extracted from the datasheet of Lumiled Luxeon C, a well-known manufacturer of LED sources available in different colours and used by many projector manufacturers in the Stage/Film sectors.
Other internal/external factors involved in the headlamp inconsistency phenomenon
Selecting LED sources with a narrow binning is, however, not an exhaustive solution to the phenomenon of fixture inconsistency, as there are other internal factors related to the tolerances of the components used and external factors that have a strong influence on the performance of the luminaire which, combined with the characteristics of the LED chips, amplify the problem of consistency. For example, some of them include:
– tolerance of the optical assemblies : as with chips, optical assemblies also have tolerances in their production process, both in terms of the materials and in the coating processes applied to optical systems. Lenses obviously have an impact on the light emitted by the source, acting on both the spectrum and the intensity emitted by filtering the light.
– tolerance of electrical and electronic components: as with chips, other components, such as power supplies and especially LED drivers, also have manufacturing tolerances.
– projector temperature: the tolerance of the cooling system components (heat sinks and/or fans), as well as the temperature of the external environment, affect the temperature reached by the projector and consequently the temperature of the LED source.
As is well known, LEDs are sensitive to heat and as the junction temperature increases, there is a decrease in the light output of the individual emitters. Taking the example of the Luxeon C chip datasheet above, it is easy to see that LEDs of different colours all have a flux decrease curve depending on the specific temperature, e.g. the colour red is more sensitive to temperature increase than lime or mint and the decrease in brightness will be steeper.
In projectors with a multi-colour source, the reproduction of white and colour is always given by the combination of the emitter colours in the source, and it is therefore easy to understand that the variation of temperature during the use of the projector will lead to both a different result between one projector and another due to construction tolerances, but above all with the same projector being used at different times or stages (e.g. between the programming phase of the projector and the playback phase of the show, or if the show is played in different locations).
Introduction to Spektra calibration technology from PROLIGHTS
The PROLIGHTS projectors marked on the website or datasheet with the Spektra mark adopt an ecosystem of different technologies and manufacturing processes that not only guarantee a high degree of consistency in projectors of the same model even if produced in different batches, but also allow projectors with different LED sources in terms of number or colours of emitters in the source to be able to reproduce the same functionalities and results with continuity and in a user-friendly manner.
Projectors equipped with this technology will therefore have followed certain production processes and will have hardware/software as described below:
Characterisation of headlamp source and optical unit
The projectors, once assembled and checked in an initial quality control, are uniquely characterised/measured for the specific source, optical unit and on-board components.
The characterisation process takes place via a calibration station consisting of a mechanical element that isolates the projector from the environment (‘dark room’) and a spectrometer, connected to a server control unit that manages the sequence of tests per single emitter colour and their combinations, performs the measurements, and collects the data that is subsequently archived and sent to the projector, associating it with its unique identification number.
Hardware on board the projector
The projector is equipped with an internal memory containing characterisation information, and a set of sensors capable of real time detection of temperature, power consumption, operating hours and fan control.
Internal fixture software
The projector’s on-board software, containing control algorithms for emitters and colour management, reads and processes the information contained in the calibration memory and combines it with sensor and projector status information in order to predict and reproduce a result that is consistent from projector to projector.
The target defined by the user control will be reached by software selecting the combination of the various emitters to find the best compromise between intensity and quality among those available.
Operating modes available in calibrated SPEKTRA projectors
The calibrated SPEKTRA projectors offer within the advanced settings the possibility of selecting different modes of operation and behaviour of the projectors within the colour space reference as described below:
Spektra Calibration ON
In this mode, all primary colours, i.e. the colours of the emitters with which the source is composed, are adjusted in order to have a colour space that is replicable by any produced unit, taking into account the tolerance ranges of the LED binning and the other factors listed in the previous paragraphs. The adjustment is done by correcting the primary colours by adding a small fraction of the other colours, thus narrowing the available colour space by considering the worst possible tolerance scenario.
In Spektra calibration mode ON, the projectors are therefore very consistent with each other in all colour control parameters, but access to saturated colour reproduction will be limited as the primary colours of the source have been adjusted by adding other colours.
Spektra Calibration PURE COLOURS
In this mode, the projectors present a calibrated and consistent operation when colour reproduction is done by means of linear or snap mixing parameters that are managed by the software’s internal algorithm.
The modes that have calibrated behaviour are: RGB/HSI/CMY, XY, Gel presets, Colour & White macros, Source emulation.
Modes that provide direct access to the primary colours of emitters (ie. RGBW) will operate in RAW mode, i.e. uncalibrated, allowing the reproduction of the saturated and pure colours that are available on the source emitters.
In this mode, the closer inwards the colour space (white colour), the greater the consistency of the projectors. Similarly, the further you move away from it and reach the perimeter of the colour space (saturated colours), you can see differences between projectors.
Spektra Calibration OFF
This mode disables the characterisation of the source (RAW mode) and all colour-related functions will be handled by the projector software, providing average values within the expected tolerance ranges. This is the mode in which luminous flux is maximised, but differences can occur in terms of both consistency and light quality parameters.
Colour control parameters in calibrated PROLIGHTS projectors
The calibrated Spektra projectors offer multiple colour control modes, in the various user modes available on board the machine. Some of the main ones are given as examples below:
– RGB/HSI/CMY: simplifies the control of a multi-colour LED source (4, 6 colours or more) in just 3 color channels, making it easier for the user to select the right combination to achieve the desired result. Very useful for common lighting consoles handling internal colour pickers or colour presets with 3ch input. Based on the three-channel control input, it is the projector’s internal software that decides on the best combination of the 4 or 6 emitters within the source to achieve the desired colour point. During colour transitions in these modes, the software also considers the maintenance of an even intensity level when switching from one colour to another.
– Linear CCT control channel: White colour reproduction on a linear White CCT on the Black Body Curve with a variable interval depending on the projector in object.
– Channel Tint: DUV adjustment +/-0.25 or correction +/- green shift in colour space on axis perpendicular to the blackbody curve to correct the tonality of white light.
– XY control: By inserting colour co-ordinates in a CIE 1931 space, a universal target is defined even for projectors with different types of multi-colour sources and thus an extremely consistent result within the intersection of the colour space common to the projectors being controlled at the same time.
– Colour Gel presets: Multiple gelatine presets from different manufacturers and libraries to emulate the colour co-ordinates obtained by filtering traditional lamp with classic gel filters. The latest Spektra calibrated products also allow spectrum emulation and traditional source selection with multiple choices between Tungsten or Discharge sources.
– Source Emulation: Reproduction of a traditional source library with emulation of CCT, spectrum and dimmer curve.
We thank you for taking the time to read this and, as always, we do appreciate your support. For further questions on SPEKTRA calibration technologies for PROLIGHTS products, please feel free to contact us.