Single sensor video cameras and the TLCI-2012

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ABSTRACT

A presentation at IBC2011 with the title “A Television Lighting Consistency Index” ‘kickstarted’ the EBU work leading to the adoption of EBU Recommendation R-137 (TLCI-2012) about one year later.

This work was mainly based upon the behaviour of modern broadcast HDTV cameras with 3-sensor technology. At IBC2012 another presentation was given describing the development of a “Standard Camera Model” used in calculations of the TLCI-2012 Qa value.

Results from measurements of luminaires using the TLCI-2012 have since been published by the EBU and in technical magazines. The EBU work activity behind R-137 was very much aware of the development and increasing interest in video cameras using a ‘singlesensor’ solution based on a colour filter array (CFA) on the sensor of a Bayer-pattern type.

At that time and for several reasons, it was difficult to make reliable measurements of the spectral sensitivity behaviour of these cameras; measurements from only one such camera was incorporated in the original work (data provided by the camera manufacturer under an NDA). Recently some of the difficulties mentioned above have been overcome and reliable measurements have been made on some different singlesensor cameras. The findings from these measurements are presented in this paper.

INTRODUCTION

Any high-quality electronic camera should be able to reproduce scene colours faithfully in order to meet the requirements of the users (the viewers). Let us take a look at some of the parameters that are important to the reproduction of luminance and colour and the difference in design that has been observed between 3-sensor/1-sensor cameras.

Going back to the successful introduction of colour TV the preferred cameras used three Plumbicon™ tubes or similar. These types of tubes were the first to have a linear transfer characteristic; the electrical output signal from the tube was directly proportional to the light level (illuminance) on the tube face.

In the 1980’s came the introduction of electronic cameras using three CCDs (Charge Coupled Device sensors); these also have a linear transfer characteristic.

To separate the three colour components red, green and blue (R,G,B) a light-splitting device (beam splitter) separates the incoming light in its red, green and blue components. To find the optimum performance for this light-splitting much work was done at an early stage (1960’s) by taking into consideration the properties of available phosphors for the display tubes. The display phosphor characteristics were standardised first for the NTSC system and later for the European PAL system.