Choosing lenses for Power LED based luminaire

Abstract

When designing a Power LED lighting fitting, choosing a optimal secondary lens is of crucial importance. The appropriate lens or reflector is determined on the basis of dimensions of luminare body, availability of adequate beam angle and price. At first glance, the task seems very simple, however, when solving the problem practically, many problems arise.

There are many Power LED lens manufacturers worldwide. Each of them is producing lenses for determined types and manufacturers of Power LED. LED are all different so that one lens can be used on only one LED. From experience gained in the last months, we can say that a significant part of these lenses are not so dedicated and the results are quite different from expectations. When measuring lenses in combination with adequate LED we focused our attention on efficacy, repeatability, consistency with the nominal data and problems with the installation.

1. Power LED properties

High efficiency and lower energy consumption compared to the classical bulbs, halogen bulbs and in last months even fluorescence lamps are leading to ever higher popularity of Power LED as a lighting source. Another, often neglected, advantage of LED is its directionality. When other types of lighting sources emit light in all directions and so intrinsically increase losses, LED have a beam angle between 90° and 120°.

Because of this the lighting fitting can be designed to radiate light in the desired direction with almost no reflective loss.

Even so, in most cases the LED beam angle is usually to wide to be used in an lighting fixture without some secondary optics reducing the angle to a acceptable value. This is usually done with the use of secondary optics, reflectors (figure 2) and lenses (figure 1), made by specialized manufacturers in vast numbers. Because of price they are rarely custom made.

Figure 1. Power LED lenses	Figure 2.  Power LED reflectors

The most common secondary optics are reflectors and Total Internal Reflection (TIR) optic. Reflectors produce very wide beams, have good efficiency (> 90%) and have a sharp beam edge.

TIR it is a compound optic that uses a combination of a central lens and TIR mirror to collimate the light from the source (figure 3 and 4).

TIR is an optical phenomenon that occurs when a ray of light strikes a medium boundary at an angle larger than the critical angle with respect to the normal of the surface. If the refractive index is lower on the other side of the boundary no light can pass through, so effectively all of the light is reflected. When light crosses a boundary between materials with different refractive indices, the light beam will be partially refracted at the boundary surface, and partially reflected. However, if the angle of incidence is greater than the critical angle, than the light will stop crossing the boundary altogether and instead be totally reflected back internally. This can only occur where light travels from a medium with a higher refractive index to one with a lower refractive index.

Light ray paths through a TIR optic / light ray paths through a TIR optic regarding incidence angle

Secondary optics are characterised by how wide a beam they produce. The angular width the optics produce is usually specified by measuring the angular separation between the directions, at which the intensity has fallen to half its peak value. The value is called the Full Width Half Maximum (FWHM) divergence.

Full Width Half Maximum Angle definition

When mounting secondary optics positioning the optics at the correct height relative to the LED is essential if you are to obtain best efficiency and the correct beam width. Equally important is the alignment of the optic axis to the LED chip. If not correctly positioned, the output beam will become uneven and offset. Although a industrial standard doesn’t exist, a commune accuracy is ± 0.2 mm.

Lens producers usually supply a range of holders for a single emitter LED, starboard mounted LED and various versions of multiple lenses. Although solving part of the problem, holders usually have the same ± 0.2 mm accuracy, so for narrow spot they might not be the perfect solution.

LED, lens, holder assembly

2. Lens comparision

In the first part we measured lens efficacy regarding the forward current trough the LED. Then we compared lenses of different manufacturers, and we tested the repetibility of lenses of the same manufacturer. At the end we tested what happens when bad holder is used and focus is moved.

2.1. Dependence of luminous intensity on the forward current

To control behaviour of the lens under different forward current, measurement was made using one lens on a LED with different forward current applied. Forward current of 700mA and 1A were used on Seoul P4 and Cree Xr-E LED. In both cases 1A is the maximum current that could be applied to the LED without damaging it. LED with its lens was installed on a big disipator and the complete set was then installed on a fotogoniometer.

In the case of Seoul P4 with 9.5 ̊ peak at 700mA is at 1998 cd, and 2520 cd at 1A. The ratio is 1.26. Ratio of the forward current is 1.43. Lower efficiency is due to the higher junction temperature. Despite a large disipator the junction temperature has risen and the efficiency droped.

In the case of Cree and 36̊ lens at 700mA peak is at 346 cd, and 436 cd for 1A. The flux ratio is also 1.26 and current ratio 1.43. Again there is a small difference which can be led to the higher junction temperature.

From this measurements can be concluded that lenses behave identically at different forward currents and when measuring them only one forward current can be applied. From there other values can be calculated, taking care to include junction temperature in the calculus. Dependance of luminous flux of LED of junction temperature can be obtained from the manufacturer.

Dependance of iluminance of the forward current

2.2. Comparison of lenses of different manufacturers

The next task was to measure and compare lenses of different manufacturers and same beam angles.

We compared lenses of three manufacturers for Cree XR-E with 8̊  FWHM. The difference between lenses are immediately visible. The highest luminance (5810 cd) is 35% higher than lowest (4310 cd). The comparision diagram is shown on figure 8.

Relative luminace with lenses (8º) of three different manufacturers (Cree Xr-E)

On the second diagram narrow beam lenses (10̊) of the same manufacturers for Seoul P4 LED are compared. Measured luminances are from 1415 cd to 2100 cd. So, for this lenses the differences are big, more than 50% between the best and worst.

Relative luminace with lenses (10º) of three different manufacturers (Seoul P4)

On third diagram wide beam lenses (40̊) for Seoul P4 are compared. Measured luminances are 278 cd, 296 cd and 332 cd. The differences are smaller but nonetheless significant.

Relative luminace with lenses (40º) of three different manufacturers (Seoul P4)

In our measurements we found that differences between the lenses manufacturers are big and that “good” manufacturer name doesn’t guarantee good lenses. In our measurements, namely, lenses form an unknown Chinese manufacturer were better than lenses form a well known European manufacturer, which is about twice as expensive.

2.3. Impact of the lens diameter

We measured the lenses of the same manufacturer but of different diameter. We expected to get better results from the bigger lens. We have to know that the small lens is a compromise between the requirements of lighting designers, who need small dimensions, and engineers who know that a optimal lighting efficiency need adequate space. We measured lenses with 20mm and 26mm diameter (which are something standard dimensions).

For a broad angle lenses (about 40̊) difference between luminance is small (296 cd and 360 cd)(figure 11). Here, we must bear in mind that the lens with smaller luminance has broader angle (2.2̊) and therefore the lower luminance at 0̊ is justified. So the light yield is practically identical.

At narrow angle (10̊) the difference is huge, 2460 cd and 1415 cd. This is 73% difference (figure 12).

Relative luminace with lenses (40º) of different dimensions (Seoul P4)	Relative luminace with lenses (10º) of different dimensions (Seoul P4)

The measurements confirm the assumption that bigger lens have greater efficiency.

2.4. The problem of lens positioning

In order to have a perfect symmetric light distribution LED has to be positioned as precise as possible. On the figure 13. a) and b) light emitted form a white object illuminated with power Led luminare is shown.

Surface illuminated with LED with incorrectly positioned lens (10°)

As it can be seen, maximum light is not in the centre of the illuminated surface. This is due to the error in positioning. First probable error is the movement of the lens, which has a small possibility of movement, about 0.2 mm. The second error arise when the starboard is screwed in the housing. It is not possible to estimate how much the LED has moved, but the displacement is minimal. The same effect can be seen but using broad angle lens (figure 14).

Surface illuminated with LED with incorrectly positioned lens (40°)

Conclusion

In this article several problems regarding secondary lenses for power LED were analysed. The biggest problem is certainly the choice of the lens. With measurements we found that different manufacturers have lenses of different quality for different power LED and different angles. We found that it is possible that a certain manufacturer has excellent narrow angle lens and bad broad angle lens. Because of this phenomenon it is difficult to determine which manufacturer has the best overall lenses. Practically, this means that to choose the lens for power LED, we have to measure all lenses and all angles and make a compromise.

As far a diameter of the lens goes, for narrow angles bigger diameter should be used. For wide angles, diameter is purely a design element.

At the end a problems arising from less than perfect alignment of lens and LED were examined. As a small non-alignment can lead to significant errors in the enlighten surface. Positioning of narrow angle optics proved even more difficult. It is advised to use specially designed holders should be used whenever possible.

references
1.     Carclo Overview-01-09.pdf
2.     http://www.glenbrook.k12.il.us/gbssci/phys/CLass/refrn/u14l3b.html
3.     http://www.ledil.fi/

Authors:

Vladimir Furlan
Intra Lighting d.o.o.
Miren 137b, Miren, Slovenia
+386 (0)5 398 44 58
Vladimir.furlan@intra-lighting.com

Matej B. Kobav
University of Ljubljana – Faculty of Electrical Engineering,
Tržaška 25, Ljubljana, Slovenia
+386 (0)1 476 87 59
matej.kobav@fe.uni-lj.si

Davor Širca
Intra Lighting d.o.o.
Miren 137b, Miren, Slovenia
+386 (0)5 398 44 54
Davor.sirca@intra-lighting.com