Understanding the characteristics of the lamps used in video projectors and going as far as to track the light output of your projector can be a useful exercise for some home theater owners. The industry standard for lamp life is based on a 50% dimming of the lamp. Therefore, when a projector manufacturer specifies that a specific model of projector has, for example, a lamp life of 2000 hours that means that they expect the lamp to lose one half of its original (i.e, when the lamp was new) light output after 2000 hours of use. However, there are other factors that can impact the lamp life and it can be useful for the projector owner to understand what these factors are where their projector’s lamp actually is in its life cycle.
The vast majority of projectors available today that are suitable for use in a home theater use mercury vapor arc lamps (bulbs) with the mercury vapor gas contained within a glass envelope. The gas is under high pressure within a gas envelope and such lamps using this technology are most frequently referred to as a UHP (U
ressure) lamp, or for certain less common variations they are referred to as a SHP (S
ressure) lamp. For the remainder of this blog I will refer to the overall class of such lamps as UHP. These are arc lamps meaning they contain no filament, as used in common incandescent or halogen light bulbs. Instead, UHP lamps require a high voltage electrical pulse to be applied to the lamp or order to create an electrical arc between internal electrodes which triggers the ignition of the lamp. The UHP lamp is a technology that was developed by Philips in 1995. Virtually all projector manufactures purchase the UHP lamp itself (i.e., bulb) used in their lamp assemblies from one of very few UHP lamp manufacturers, including Philips, Osram, Ushio, Iwasaki, etc.. The lamp assembly, or module, used within a projector includes a housing that contains a UHP bulb that is placed in front of a curved (e.g., parabolic) mirrored reflector. This reflector, placed toward the rear of the lamp assembly, is used to focus the light which exits the lamp assembly through a clear glass window in front of the bulb. The two photos below show these part of the lamp assembly (photo is of a JVC lamp assembly).
Lamp Assembly - Photo 1
Lamp Asssembly - Photo 2
How Lamps Age and Fail
The light output of UHP lamps will decrease over time. The light output will be the greatest when the lamp is new and will typically lose on the order of 20% of the original light output over the initial several hundred hours of use. After that the rate of loss of the light output will typically slow down during the remaining nominal life of the lamp. Ideally a UHP lamp’s light loss will be gradual and well behaved in reaching one-half of the original light output after a few thousand hours of use. The lamp life for the UHP lamps used in consumer front projectors is typically rated by the projector manufacturer somewhere between 2,000 hours and 4,000 hours. This rating is for a 50% loss of the original light output. Most projectors will have a high lamp and a low lamp setting (labeled various names by the different projector manufacturers). Many manufacturers will rate the lamp as having a longer operating life when the projector is operated in the low lamp mode.
While high vs. low lamp mode can impact lamp life there are other significant factors that should be considered. These are the number and frequency of on/off lamp cycles and having adequately cooling for the lamp.
Lamp life is reduced each time the lamp is turned on. Also lamp life can be shortened by not allowing adequate time for the lamp to fully cool down after being turned off before being started again. While I have not seen any scientific study to confirm the specifics, there seems to be a general opinion among many experienced users that to achieve maximum lamp life, at least a 30 minutes cool-down period should be allowed before turning the projector back on again. Thus if you are temporarily not going to be watching video from your projector but you expect to be watching it again within thirty minutes, it is probably best to leave the projector turned on. The bottom line is that your viewing habits and the way you operate the projector have the potential of shortening the lamp life by cycling the lamp on and off more frequently that what has been assumed by the projector manufacturer in coming up with their ratings for lamp life.
Providing adequate cooling for the lamp (and other elements of the projector’s light engine) starts with the projector manufacturer providing an adequate thermal design to ensure the heat produced by the lamp is adequately vented away from the projector’s light engine. It is the responsibility of the projector owner to ensure the projector is mounted such at airflow from the projector’s inlet and outlet air vents are not impeded, the projector’s air filter is kept clean, and if operated at high altitudes the projector is configured for high altitude operation (i.e., which increases the speed of the projector’s cooling fan). Also the projector’s cooling fan will normally continue to run for a short time after turning the projector off. You should not remove power (e.g., unplug the power cable) from the projector before the cooling fan has finished the cool down period.
Sooner or later all UHP lamps will fail and such failures can generally fall into one of three cases:
Case 1. Slow graceful aging where the lamp’s brightness drops to 50% of its original light output at approximately the manufacturer’s rated lamp life (or even longer). In this case it usually just becomes the projector owner’s decision as to when the projected image has become too dim and it is time to replace the lamp.
Case 2. Gradual dimming followed by a very rapid dimming. This failure mode frequently occurs after several hundreds (or more) hours of gradually dimming of the lamp (as typical for case 1 above). In this case this can be one mode for a premature lamp failure which is typically cased by a defective lamp. However, if this were to occur after a few thousand hours of normal bulb dimming it would not necessarily indicate an abnormal failure of the lamp. In either case, this failure mode is characterized by the lamp going from having a nominal light output then all at once the lamp very quickly dimming by 50% to 90% over just a few hours, to a few tens of hours of use.
Case 3. This failure mode is a catastrophic failure where in the worst case the UHP bulb itself explodes. If you are lucky the lamp will simply fail to start or if the bulb does explode the best case is where the shattered pieces of glass are contained by the lamp housing. If the worst really happens with a bulb explosion, you will be cleaning pieces of glass out of the projector before inserting the new lamp assembly. Luckily, this is probably least common failure mode, but it certainly still happens all too frequently.
Tracking my Projector’s Light Output
Projector manufacturers provide warranty coverage for the lamp which may be for a shorter time period than for the projector itself. Some manufacturers place both a time and “hours-of-use” limit for warranty coverage on the original lamp installed in a new projector. For those projector owners that experience dimming of the lamp while the lamp is still under warranty, it can be useful to have hard data as to how much dimming has actually taken place. Again, the industry standard is a 50% decrease in light output represents the end-of-life for the lamp. For example, if your new projector has a 1 year warranty on the lamp and after just 10 months you have data to show that the lamp’s light output has decreased by 60% and you have not exceeded any hours-of-use-limit stated in the warranty, then you should have justification to request a replacement lamp from the manufacturer.
Some hobbyists have begun tracking the light output of their projectors in order to understand how well, or poorly, the lamp is aging. This has become more practical over the past few years as suitable light meters have become much more affordable. This type of light meter measures the light level in units of lux. Technically ‘lux’ is a measurement of luminous flux per unit area (Wiki for a detailed explanation is HERE
Pictured below are two different models of economical lux meters (the one pictured on the left typically sells for under $100 and the one pictured on the right sells for under $50). Both of these models are sold under several different brand names from such on-line sites as Amazon and eBay.
Two Inexpensive Lux Meters
Such inexpensive lux meters may not have high absolute accuracy (their accuracy may vary by perhaps 10% from what would be measured with an expensive professional light meter), but the important thing is their relative accuracy from one measurement to the next (perhaps taken months or even years apart) should be adequate to track the dimming of the projector over time, as the lamp ages.
A projector’s light output is normally specified in lumens and this value is independent of the screen size or gain. Lumens is a measure of the total amount of visible light emitted by the projector (Wiki for a detailed explanation is HERE
). For projectors that are two common methods in which the lumens are measured and specified. In either case, a 100% white test pattern is input to the projector.
are measured at the center of the projected image (i.e., at the center of the screen). This is typically the brightest point of the projected image.
are the average of measurements taken at 9 points within a 3 x 3 grid across the projected image (as illustrated below). This is a standard measurement technique defined by the American National Standards Institute and gives a better indication of the overall image brightness as it provides an average across the projected image rather than just a measurement at the brightest point. Many projector manufacturers specify their projector’s light output in ANSI lumens. However unless the lumens value in a manufacturer’s spec sheet specifically indicates the value is based on the ANSI standard, then most likely peak lumens (which will normally be a higher value than ANSI lumens) are being used.
If all you are attempting to do is to track how much your projector’s lamp is losing light output as it ages, then a simple peak lumens measurement should suffice.
The correct approach for measuring a projector’s light output and to track any changes over time is to use the exact same conditions for each measurements. Below are my recommendations:
1. light meter capable in measuring in units of lux (i.e., lux meter)
2. DVD or Blu-ray Disc player connected to the projector
3 DVD or Blu-ray test disc that includes a 100% white screen test pattern. Commercially available test discs such as the Digital Video Essentials disc (DVE is available in both DVD and HD-Blu-ray Disc versions). Also there are freeware test discs available as image files that can be downloaded to a PC and then burned onto a DVD (or Blu-ray Disc). One such freeware image file is the AVS HD Calibration Disc (ISO file that can be burned on a DVD +/- R or a BD-R) that is described and can be downloaded HERE
. Once downloaded you can use a commercial program such as Nero, or a freeware program such as IMGBURN
, to burn a disc playable in most Blu-ray Disc players.
4. I have prepared an Lumens Calculator spreadsheet file (for use with Microsoft Excel), that can be download from the link below, that lets you enter your screen size, and your measured lux values then it will calculate the projector’s lumens output. It can be used to calculate either peak lumens or ANSI lumens based only our lux measurements with the light meter. Download this file and run it with an Excel compatible spreadsheet program on your PC.
Link to download the Spreadsheet: Lumens Calc
Once you have the test tools in place it is time to actually perform the measurements. The procedure is as described below:
1. The projector must be set to the same user setting for the all measurements that are to be made over the life of the bulb. You can use one of the factory presets for this, such as vivid or dynamic picture mode with the settings (e.g., contrast, brightness, etc.) at their factory default settings and with the lamp set to high output mode. This combination is normally the brightest factory preset mode, but also the least accurate in terms of grey scale and color. In any case, in order to be able to compare your initial measurements for the projector’s light output with later measurements, the projector must be operated under the same conditions each time a measurement is being made.
2. Play the test disc and project the 100% white test pattern (you must use a full screen test pattern if you are going to do an ANSI lumens measurement).
3. Using the Excel Spreadsheet that I have provided (see download link below), record the diagonal size and aspect ratio of the image. This image size is typically the diagonal screen size (in inches) plus perhaps 2 to 4 additional inches to allow for the overscan of the image onto the black borders of the screen. Note that all numeric units on this spreadsheet are in English units rather than metric. Most modern HD projectors have a native 16 x 9 aspect ratio (or 16/9 = 1.78 aspect ratio which is the default value for the spreadsheet). As explained in one note on the spreadsheet, while it is not essential for the lumens calculation, you should also enter the horizontal throw distance (in inches) from the screen to the lens of the projector.
4. The room must be very dark with all light sources, other than the projector, turned off or blocked out (e.g., windows). If you cannot fully block the light coming through any windows in the room then I suggest you perform the measurements at night.
5. The light meter should be set to measure in lux (many such meters also give the you the option of measuring in foot candles) and if there is a peak vs. average reading setting (or fast vs. slow measurement setting) then in most cases the average or slow setting will work best. Finally you should set the meter range to allow measurements of up to perhaps 1000 lux as a starting point then change to a lower range if the initial lux measured value falls within the next lower range on the meter. For example, if the specific meter has one range for 0-to-200 lux and the next higher range is 0-to-2000 lux, then start off with the 0-to-2000 range and if the initial measurements fall below 200 lux then switch to the 0-to-200 range for the current and future measurements.
6. The lux meter probe has a white dome behind which is the light sensor. The back of the probe should be placed very close to the screen surface with the white dome facing toward the projector.
7. If you are only wanting to measure the peak lumens, then place the probe at the center of the screen and record the lux reading from the meter into the Excel spreadsheet. If you cannot run the attached Excel spreadsheet on your computer, then see the note below for how to manually calculate the lumens.
NOTE: Manual calculation of lumens. If you do not have an Excel compatible spreadsheet program on your PC or Mac then you can manually calculate the lumens once you have made the lux measurement by using the following formula:
Lumens = (Image Area* in square feet) x (Measured Lux) x (0.0929)
* The image area is simply the width of the image times the height of the image measured in feet. For example the viewable portion of a 100 inch (diagonal) screen with an aspect ratio of 1.78 (regular 16 x 9 HD aspect ratio) is approx. 87 inches wide by 49 inches high. If you allow the image to extend approximate one extra inch onto the right and left black borders of the screen this will produce an image that is approx. 89 x 50 inches with an area of approximate 89 x 50 = 4450 square inches which is 4450/144 = 30.9 square feet.
8. If your want to perform the ANSI lumens measurement, then use the lux meter to take 9 readings with the probe placed near the center of each of the 9 rectangles as shown in the figure above. Record these 9 lux readings in the Excel spreadsheet.
9. Create a log and record the number of hours on your projector’s lamp and the measured lumens value (calculated using the attached Excel spreadsheet). The hours of use on the lamp can usually be accessed through the projector’s user menu. For example, my Epson projector displays the lamp hours under the user “Info” menu and my JVC projector displays the lamp hours under a similar user “Information” menu. While with most projectors this can be found under an info menu directly available to the user, with some other projectors this information may only be found in a service menu. Check your projector’s owner’s manual or Google for how to find this info for your specific projector model.
By repeating the same measurements, under the same conditions, at different points in time (e.g., after every 200 hours of time on the lamp) you can track the light output of your projector and thus determine to what extent your projector’s lamp is dimming over time. If you measure a sudden drop in lumens or you find that the projector’s lumens output has dropped below 50% of the original (with a new lamp) value, then it is time to consider replacing the lamp.
While there are now a few home theater oriented projectors that use LED or LED/Laser hybrid light sources, rather than UHP lamps, lamp-based projector are expected to dominate the market for at least the next few years.
I hope that some projector owners will find the above information useful.