Today, light emitting diode manufactures market their products for LED lighting fixtures with a strong focal point on cost efficiency. Lumens per dollar defines overall LED lamp cost efficiency. A light emitting diode offering 20 lumens per dollar can produce more light for fewer dollars than a device offering only 10 lumens per dollar. In a simple scenario such as this, the choice is clear. The 20 lumen per dollar device can obviously manufacture twice the amount of light for the same cost, and becomes the prime chose for any custom LED lighting fixture. However, this may not be entirely true in some extreme cases. In reality, the LED lights with lower cost efficiency actually turn out being more efficient. How can this possibly be?
The concept of lumens per dollar is in reality, extremely vague! To understand why, one must consider the big picture by adding a third or even fourth factor to the custom LED lighting fixture. Think multi dimensional, and you can begin to understand the forgotten aspects associated with the lumens per dollar concept. The commonly overlooked factor of "time" plays a critical role in the lumen per dollar overall concept. This may be due to the manufacture's unavailability of technical information or even their intentions to present it. When a manufacture states their product can produce 20 lumens per dollar while utilized within LED lighting fixtures, the information is meaningless unless they mention for what period of time the device can perform in this manor. Another significant factor is order volume. When a manufacture advertises 20 lumens per dollar on their website homepage, that value may only apply to those customers purchasing greater than one million pieces annually.
LED manufactures typically bin or separate LEDs subsequent to a production run. The factory tests each LED light for specific characteristics such as luminous intensity, forward voltage, and optical wavelength or color temperature. Consider a low intensity bin that starts at 15 lumens, and a high intensity bin that ends at 50 lumens. Depending on the bin, LED lights can produce anywhere between 15 and 50 lumens. As some manufactures of LED lights allow the customer to specify a bin before purchasing, while others do not. This is why it is extremely important to consider the company's ordering procedures. If the customer cannot specify a bin prior to ordering, the LED light manufactures is free to ship only LED lights from the lowest intensity bin! In any case, it is also important to check factory yields. In some cases, the LED manufacture may allow the customer to specify the intensity bin. However, they may claim that factory yield for the high bin accounts for a very low percent of their LED production. Therefore, the high intensity bin is usually not even available for ordering, even when it is specifically required. A marketing group may decide to advertise the highest intensity or lumen value from the highest intensity bin. In reality, they should be using the typically value from the intensity bin providing the highest factory yield. Perhaps binning is the single most misunderstood aspect related to inaccurate values associated with lumen per dollar calculations.
While designing custom LED lighting fixtures, drive current is always an important consideration associated with the lumen per dollar concept. This is because increased currents result not only in increased lumen outputs, but also in increased heat dissipation and decreased lifespan. In an extreme case, consider an LED manufactures who states 12 lumens per dollar based on a forward drive current of 60 mA. At this current rating, the device may perform for only 50 hours before degradation results in a noticeable loss of luminous output. Although the lumen per dollar rating is not necessary a lie, it represents an unreasonable value based on an overdriven forward current rating. Driving the device at the recommended typical rating of 20 mA might provide a much lower lumen per dollar value, but will result in a much more reasonable lifespan. This example shows why it is always important to examine drive currents associated with lumen per dollar values before considering the device for integration into an LED lighting fixture. Real-life examples may include cases with lumen per dollar value based on the typical forward current rating. However, the light emitting diode may be exposed to an environment with a higher than normal ambient temperature, usually greater than 25 degrees Celsius. Therefore, derating the device is necessary to prevent from overdriving according to the temperature-derating curve.
Perhaps the most common and abused lumen per dollar deception is based on the manufacture's actual order requirements. Since lumens per dollar is directly affected by cost, it is tempting for an LED manufacture to advertise cost efficiency based on pricing for a ridiculous order such as several million pieces. Smaller companies or individuals may not possess enough buying power to support the hundred thousand-piece price break. Once again, consider the previous example where an LED manufacture is advertising that his LED lamp can provide 20 lumens per dollar, within your custom LED lighting fixtures. Assume the cost of each LED is $5.00 if the customer can commit to a purchase of 100,000 pieces. Therefore, the statement should truly state that 20 lumens per dollar is offered with a minimum purchase of $500,000.00. In lower volumes, the LED lamp may cost $10.00 each, decreasing the cost efficiency to 10 lumens per dollar. This is why it is so critical to research lumen per dollar claims based on order volume, before just assuming the component truly offers the cost efficiency advertised.
Overall lifespan should be a major factor while considering lumens per dollar, for your custom LED lighting fixture. Even though some LED lights may accompany superior lumen per dollar figures, they may lack in terms of longevity. The result is a lumen per dollar value that remains valid for a very short period. As all LEDs age, they will suffer from degradation as apparent by a reduction in luminous output. Certain LEDs may suffer more so than others. In our previous example, we considered two devices, one rated at 20 lumens per dollar and the other at ten. Suppose the 20 lumen per dollar device will present a 50% loss of luminous output after 10,000 hours of accumulative operation. However, the 10 lumen per dollar device can operate out to 25,000 hours before reaching a 50% luminous output. In this case, the 10 lumen per dollar device offers increased cost efficiency when applied in an application requiring at least 25,000 hours of accumulative use. This concept proves the importance of demanding lifespan data from LED lamp manufactures.