DATA PROJECTOR

Introduction

With the change from film to digital technology the data projector has replaced the slide projector for the display of projected images.

Northside Creative Photography purchased a data projector early in 2006. This exercise proved to be very challenging. A data projector is an expensive acquisition and hence the selection of a suitable model for camera club or personal use is critical. It was found that the majority of XGA data projectors were targeted for the business presentation market where brightness and high colour temperature are the most important parameters. Very few placed an emphasis on the accurate display of photographic images.

After an exhaustive evaluation, Northside Creative Photography was only able to find two models which satisfied our demanding requirements for photographic colour accuracy. These were the Mitsubishi XD460 and BenQ PB6240. Our final selection was the Mitsubishi XD460 since although it was more expensive it had greater sharpness which added a 'three dimensional' quality to the images.

The purpose of the article is to share with the photographic community our selection criteria, the process used for selection and the final choice of model. We do not consider ourselves subject matter experts and the information should be used with caution. Your feedback is most welcome since it will allow the article to be refined.

Process

The process used was to firstly establish a set of data projector technical requirements and these are outlined below. The manufacturer's specifications were then checked for compliance with the technical requirements and this resulted in the elimination of a large number of models.

Demonstration data projectors were then used to evaluate performance. To check colour accuracy a set of known images was used since any colour inaccuracies were then obvious. Calibration images were also used to ensure that there was good tonality from the whites to the blacks in all colours.

The eye is very good at carrying out relative measurements but very poor with absolute measurements and hence side by side evaluation of data projectors is critical. The services of East Coast Audio Visual were used since they had a projection room with a video splitter which allowed up to three projectors to be compared simultaneously.

The use of data projectors for photographic image display where accurate colour fidelity is important is a very small segment of this market. It was therefore difficult to obtain guidance and product reviews.

Technology

There are mainly two competing technologies in the market.

In a 3-panel LCD (3LCD) projector, light is separated into red, green and blue beams by stationary mirrors. Each colour passes through an LCD panel which creates shades of that colour. Single colour images are combined by a prism to create a full colour image in the projector which passes through a lens onto the screen. A 3LCD projector has the advantage of no moving parts in the light control system. Manufacturer’s claim 3LCD projects a full colour image with smooth motion and no colour break-up. There are many manufacturers of 3LCD projectors and Epson and Sony use this technology exclusively.

In 3LCD projectors the light is a continuous stream and proponents claim that this is more "natural" and easier on the eyes.

The showstopper with 3LCD projectors is that contrast ratios are typically 400:1 or worse. This equates to a poor tonality from brilliant whites to jet blacks and lack of vibrancy in colours. None of the 3LCD projectors evaluated were considered to have a satisfactory image quality.

In 1-chip DLP projectors the light passes through a rapidly spinning colour filter where it is broken down into sequences of red, green, blue and white. The image is formed by each colour in turn reflecting off thousands of tiny hinged mirrors through a lens. It projects a rapid sequence of red, green and blue images that are combined by the viewer’s brain to create the impression of a full colour image. 1-chip DLP projectors have the disadvantage of having moving parts which may impact on reliability. The manufacturer of DLP is Texas Instruments and the technology is used by a large number of manufacturers.

Early DLP projectors were renowned for producing a visual artifact known as the rainbow effect. More modern two speed colour wheels double the projected image rate and this has almost eliminated the problem. It is difficult to assess the impact of this factor because some people show a sensitivity to the effect which others never observe. With our projector there has been only one report of the effect being observed.

The feature of DLP projectors is their high contrast ration which is typically 2000:1.

Resolution

Data projectors have a native resolution set by the display technology hardware components e.g. 3LCD or DLP. Good quality image display demands that the native resolution of the projector matches that of the images. Data projectors generally upscale or downscale to display different image sizes but this must be avoided since the image re-scaling results in loss of image quality.

Current standards for image size and hence projector resolution is XGA (1024 x 768 pixels). This is an aspect ratio of 4:3. Notebooks are generally used to drive the data projector and the notebook and projector screens should be synchronised. Since 1368 x 768 pixels is the most common size for notebooks this works well with the XGA standard.

The next standard for image size is likely to be the widely supported WXGA 1280x800 pixels. Note this will require a notebook which has a vertical screen size of at least 800 pixels.

Home theatre projectors must not be used. These have a native resolution of 1920x1080 pixels which is an aspect ratio of 16:9. To display XGA 1024x768 pixels the projector needs to rescale with loss of image quality. In addition home theatre projectors are often not able to maintain aspect ratio and the images will have elongation distortion.

XGA data projectors will also display video in widescreen 16:9 format in pillar box mode with the top and bottom of the screen blackened out. This could be useful for camera clubs for showing training videos or audio videos converted to DVD format. For home use it allows the data projector to double up as a home theatre unit for connection of VHS and DVD players.

Colour Space - sRGB Preset

Computer monitors remain attached to the same computer and hence calibration is necessary and practical. While it is possible to calibrate a notebook and projector combination, in practice this is not feasible because projectors are often used with a variety of notebooks. If calibration is carried out it is recommended that the projector settings (contrast, colour, hue etc) be left in neutral mode and the colour profile alone be used since this will still allow the projector to be swapped between different projectors.

The above illustrates the necessity for the projector to have an sRGB preset. This is a factory calibration which sets the gamma to 2.2 and the colour temperature to 6500^oK locking out contrast ratio, colour, hue etc controls. This leaves brightness as the only adjustment to be made.

Home theatre projectors should therefore not be used. These comply with motion video standards and know nothing about the still photography sRGB standard.

Focusing

Unlike slide projectors, data projectors do not require continuous re-focusing and hence manual focusing is adequate.

Keystone Correction

If the projector is at an angle to the screen, keystone correction allows the trapezoid image to be squared up. Some projectors allow both vertical and horizontal keystone correction although the latter is not all that useful. Keystone correction may be either manual or automatic. Since keystone correction is generally digital rather than optical to maintain the highest quality image it is preferable to physically align the projector to the screen. Unlike slide projectors, for data projectors the optics is designed such that the projector should align with the bottom of the screen.

Inputs/Outputs

Absolutely essential is a 15 pin VGA input for connection to a computer. In a camera club a second input is useful since two computers can be connected with instantaneous change from one to the other. If a notebook is used it has its own screen which is synchronized to the projector display. If a normal computer is connected a 15 pin VGA pass through output is useful because it allows simultaneous connection of both projector and monitor.

DVI-D inputs are theoretically capable of providing higher quality but are not recommended. The reason is that typically the data projector may be some distance from the notebook (and presenter) and VGA allows a connection distance of up to ten metres. By contrast DVI-D cables must be short.

For connection of video most projectors will provide composite video in (2 audio and 1 video), component video, S-video and more recently HDMI.

High end projectors may provide Ethernet or wireless for connection to a network. Some have USB for connection to the notebook. This allows the remote control to be used as a mouse to allow a presenter to click through a presentation.

Home theatre projectors should not be used. These are designed for the connection of video set top units and video players with only the more upmarket models having a 15 pin VGA input.

Projection Distance and Zoom

In a camera club a long zoom allows the projector to be positioned back from the screen out of the way and reducing the requirement for keystone correction. At other times and especially for home theatre a shorter zoom is useful. In practice projectors generally do not have a wide zoom range, 1:1.2 being very common. Unfortunately most projectors are designed for the board room with typical projection distances of several metres. Only the expensive models were found to have an adequate zoom ratio and even then it often involved the purchase of an optional zoom lens. Charts are provided which allow the projection distance to be calculated. For a 1.5M wide screen it is typically around 3 metres. Manual zoom control is adequate.

Brightness

As indicated above, if the notebook/projector are set to sRGB then the only adjustment needed is the brightness control. This is a function of screen size and is a square law function. The best way to set the brightness is to display a calibration image. If the highlight squares are blown out then the brightness should be reduced, if it is not possible to distinguish the darker squares then the brightness should be increased. For the Mitsubishi XD460 it has been found that with the projector in normal power mode the brightness is suitable for an auditorium screen, for a smaller screen low power mode is perfect.

Brightness is measured in ANSI Lumens. The required brightness of a projector depends on the amount of ambient light and the size of the screen. Assuming the lights are dimmed a little and the screen size is the average of 175cm x 175 cm then 2,000 lumens should be sufficient. If the room is darkened as would be the situation for showing photographic images then 1,500 lumens should be sufficient.

Note that when a projector is placed in sRGB mode it is normal for the brightness to decrease.

Brightness uniformity which is the nemesis of slide projectors is a measure of the brightness uniformity across the screen. A figure of 90% is common.

Contrast Ratio

The higher the contrast ratio the better since this is the parameter which ensures black blacks, white whites and a good tonal range of colours. If the data projector operates in total darkness then a contrast ratio of at least 800: 1 is required.

Contrast ratio is one area where DLP projectors have enormous superiority over 3LCD projectors. A typical DLP projector has a contrast ratio of 2000:1 or better whereas a good 3LCD projector does not exceed 400:1. In the home theatre market 3LCD projectors having 800:1 contrast ratio are now common but these are currently not available in XGA projectors.

The technology of choice is therefore DLP.

Projector Lamps

A projector lamp is considered to be at end of life when the brightness has fallen to 50%. The life of a lamp is typically 2,000 hours in normal mode and 3,000 - 5,000 hours in low power mode. At the end of life some projectors give warnings while others shut down until the lamp is replaced. The projector is able to display accumulated lamp usage. For camera club use where the projector is only used for a few hours a month, lamp life is not a consideration. Projector lamps are expensive, typically around $500, which means that keeping a spare lamp is an expensive proposition. A common cause of failure is not letting the projector complete its thermal shut down cycle by cutting the power while it is still running.

Feedback welcome. Last updated 20/06/11.