Not all displays are created equal. In this article, the UK Displays & Lighting Knowledge Transfer Networks Chris Williams, levels the playing field BEINGABUYER is easy, isnt it? You get a shopping list of components and go out and buy them from your suppliers at the lowest price: easy. On the other hand, is it possibly more complicated? Maybe the purchasing professionals contribution to a companys bottom line profit isnt luck but is borne from years of experience and building professional relationships around the industry.

Instead of being involved at the concept stage, the first most buyers know about a new product is when a bill-of-materials lands on their desk with three-weeks notice to kit up a preproduction run of 200pcs at volume pricing. Buyers are expected to work miracles, blamed for buying components that fail and seldom given credit for the contribution they make to the bottom line which they could increase if only they had access to components acceptable performance requirements.

Under pressure

Buyers are under pressure to reduce unit cost and will source wherever they can to achieve that end. Unfortunately, this exposes buyers to manufacturers indulging in specsmanship where figures appearing in a products datasheet are presented and manipulated to appear better than their rivals. Also, the information may not relate to the products intended application.

A components critical parameters vary from type to type, but for displays it is always what you see is what you get. The human eye is a remarkably sensitive critic and will detect even minor blemishes in the optical appearance of a display related product.
A good example of problems caused by inaccurate and inappropriate specmanship involves LED lamps and displays. LED components can be cheap. However, colour and brightness matched LEDs are proportionately more expensive as only a small area of an LED wafer will yield chips for lamps with matching brightness and colour hue. The process requires the intervention of automated testing to select components within dedicated parameters: this takes time, equipment and personnel. A lamp used as an indicator is probably of little consequence. However, with an array, the eye immediately picks up discrepancies. Perhaps working in displays for years makes me hypersensitive, but all I can remember of the LED signboard at Manchester Piccadilly are the seven faulty pixels and I have no recollection of Moulin Rouge at the cinema due to three dead digital projector pixels.

Another good example is that brightest is not necessarily best. The key parameter for the eye is contrast and again, manufacturers data sheets are little help in assessing suitability for purpose. Have you noticed that flat panel TV sets are now claiming contrast ratios of 2,000:1, even 8,000:1? Black print on white paper has a contrast ratio of around 10:1 in ambient light. In a darkened room this drops to zero. In bright direct sunlight the figure becomes completely meaningless as reflected glare renders the text unreadable.

Unless all manufacturers specify identical test conditions for their measurements the figures are completely meaningless and unless they are traced to source or verified by an independent test house, are likely to be a work of faction from the marketing department.

Take contrast, it is a simple mathematical ratio between the brightness (or luminance) of a segment that is on and the brightness of a segment that is off. So, if a display, say a TV, is quoted with a screen brightness of 450 nits (cd/m2) and a contrast of 2,000, the off segment has a brightness of 450/2000 = 0.22 nits. Seems okay? However, this is where specmanship can make the customer look stupid.

The contrast is actually equal to total brightness of the on segment/total brightness of the off segment. We need to add the light reflected off the front of the screen from the ambient illumination. When the display is used in a normal room with lighting the contrast tumbles down. In the above case, a TV in a typical illuminated room (about 1,000 lux) sees about 30 cd/m2 ambient light reflected off the screen. Thus the actual contrast is now 450+30/0.22+30=~16! A contrast of 16:1 is a bit less than the original 2,000:1 quoted.

Reflection control is critical in good display design and good solutions cost money. How are buyers supposed to know if a commercial display is good or bad when the parameters they really need, such as front screen reflection, are not mentioned on any manufacturers datasheet? Problems dont just stay at specmanship on the data sheet.

Assured availability

Each display technology has its own idiosyncrasies. For example, LCDs can exhibit widely different optical characteristics depending on the display cell contents, surface films, ancillary backlight components and internal colour filters. Apparently similar products (according to the data sheet) from different manufacturers can look completely different when compared side by side.

One exacerbating problem with larger, more complex LCD panels is that products available in Europe can be overruns from Far East volume production in the computer and TV markets. The UK (and Europe) is mostly powerless to control the supply of such LCDs. Assured availability of any LCD panel design is typically around 18-months. There may be a replacement when the original becomes obsolete, but it is unlikely to have the same optical or electrical characteristics, screen dimensions, mechanical fixing points, connectors or even connector positions. The new products improved LCD technology cannot compensate for obsolete inventory or the re-engineering time necessary to replace the old design. This problem is completely outside the buyers control and needs to be addressed at engineering level to accommodate future products.

Buyers cannot be expected to know everything about the wide range of components they are responsible for purchasing, but help is at hand. The DTI has recognised that business support to industry must come in many forms, and they have recently established a series of Knowledge Transfer Networks (KTNs) to help address day-to-day problems that UK business faces. Much of the KTNs work will be addressing the challenges of innovation, finding and using new technologies, implementing best practice measures and reducing material costs. They will also work to help establish guidelines for business to avoid the costly errors that incorrect and inappropriate supplier data can force on them.

Display Knowledge Transfer Networks

The UK Displays and Lighting KTN is planning to produce a crib-sheet of leading questions which companies can ask their suppliers when specifying displays and lighting components. These crib-sheets will be free of charge to members and will identify the critical parameters that should be identified by the supplier. They will also explain the impact that variations in the specification might have on device performance in the application.

Meet the buyer

To create a library of crib-sheets for different applications, UKDL would like to meet buyers to exchange views. Feel free to contact the organisation to discuss your particular applications and problems you have faced. UKDL will work with you to develop a crib-sheet.

The UK Displays and Lighting KTN offers members a range of activities designed to support business and personal development. These include tutorials, workshops, seminars and training events. The organisation is structured to ensure it remains a KTN run by industry, for industry. Membership is free, following registration on the website.