Projectors: the key indicators of image quality15 April 2015
In the first part of this projectors feature we looked at what the spec sheet doesn’t tell you and how difficult it is to accurately ascertain the quality of a projector. Here we find out what impact the environment a projector is used in can have and highlight some elements that are often overlooked but contribute a great deal to the image quality.
Where there is greater consistency and ability to rely on the specification is in the area of resolution. A pixel is a pixel is a pixel, after all. Once again, as with brightness and contrast ratio, it’s all about the environment and the application. Even here, things aren’t as straightforward as they may seem.
“Should a projector be mounted such that keystone correction is required,” explains David Close, products and applications manager, EMEA at Digital Projection, ”then, right away, optimum quality is beginning to be eroded purely because the physics of distorting an image to make it fit the screen will either throw away data or invent data that wasn’t there in the original image.”
“In my mind, resolution is the key characteristic in the quality of a projected image,” says Colin Boyle, product marketing specialist – projectors at Canon Europe. “However, the quality of the projected image really depends on the resolution of the source material. For example, if the projector has a resolution less than the source material, the number of pixels used to create this will be lowered, compressing the quality of the image being projected.”
Gerd Kaiser, product line manager, large-venue projectors at NEC Display Solutions develops the theme. “Internal signal processing quality is also very important,” he notes. “High resolution is meaningless if low-bandwidth signal processing is delivering poor picture source or ‘noise’.”
There is an element of projector design that is all too easily overlooked.
“The industry has long focused on higher contrast ratios, increased resolution and higher brightness,” says Boyle. “However, without suitable lens quality, it’s impossible to take full advantage of such advancements.”
There is also the issue of the extent to which image quality can be maintained over time. Here, too, there are several factors at play – perhaps the most important of which is the illumination source.
“LED technology offers consistent brightness and colour performance, with minimal degradation,” says Justin Halls, head of product marketing at Optoma Europe. “This means they will maintain their out-of-the-box image quality without the need for expensive lamp changes or recalibration throughout their lifetime.”
Paul Wilson, product manager for visual communications at Epson UK, notes that home theatre users, in particular, may have more demanding requirements.
“A projector that features ISF Certification meets all the requirements of the Image Science Foundation in terms of what adjustments can be made,” he points out.
Brightness, contrast ratio, resolution and lens quality may be critical to projected image quality – but so too are source quality and resolution, transmission rates, signal processing electronics, how easily dust and dirt accumulate on (and can be removed from) the optics, the extent to which adjustments can be made, the illumination source – and that’s without taking into account external factors such as screen quality, ambient light and so on. It can even derive from such apparently esoteric considerations as to how many segments the colour wheel of a 1-chip DLP projector has.
The industry seems unanimous. While the data sheet may give a high-level indication of possible projector performance – assuming that the manufacturer is using the same standards for measuring brightness and contrast for instance, which is not always the case – it is far from telling the whole story when it comes to image quality. The data sheet cannot objectively quantify the performance of a lens, or the accuracy of the colour palette – nor yet the quality of the image-processing electronics, all of which make a significant contribution to image quality. It can tell you nothing about colour saturation or realism. Additionally, there is the question of the extent to which a manufacturer provides the tools to calibrate a projector and optimise image quality – and the need to compensate for possible deterioration over time. And: all of this needs to be placed in the context of two key variables – the application and the installed environment.
The conclusion couldn’t be clearer. There is no substitute for comparing candidate projectors in the proposed location. Seeing – not reading – is believing: it’s the only way of verifying that the image quality ‘promised’ by the data sheet is the image quality that will be delivered.