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L I G H T I N G D E S I G N& A P P L I C A T I O N

Light measurement performance

Light performance is very important as there is little point in improving energy efficiency and overall costs if the required lighting intensity has not been met. For the purposes of this test, the light level required is an average of 600 lux at a height of 0,85 m above ground. The height of the light fittings is 3 m and the measurement height is 0,85 m from the ground.

70 W metal halide

Fig. 2 shows a typical store with 70 W metal halide lamps spaced 2,5 m apart.This was also rendered in Relux (professional lighting design software) with a spacing of 2,5 m between the 70 W metal halide fittings. The size of the shop is 200 m² with a ceiling height of 3 m and the measurements at a height of 0,85 m.As seen in the Relux output, the average light output is 545 lux when using a 70 W metal halide at a spacing of 2,5 m.

Comparing 70 W metal halide with LED down lights

by Cornel Neethling, LED Lighting

The purpose of this test is to determine how many 8-LED or 15-LED down lights are needed to achieve an average of 600 lux at a height of 0,85 m above ground. The goal is also to do a cost comparison between the 70 W metal halide light fittings and the cost during the lifetime of the 8-LED or 15-LED down lights.

Real life measurements

Real life average measurements of these stores at floor level are captured in Table 2. These measurements are affected by different light sources from all over the shop. These figures are shown below because often the actual readings in many stores are different

from what is originally specified. This is mainly due to lumen depreciation of the light sources over time.

LED measurements and comparison

Real life measurements were taken to obtain a comparison between the LED

Test model 8-LED down light 15-LED down light 70 W RAD metal halide

Light source 8 x 2,3 W SSC P4 15 x 1,2 W SSC P4 1 x 70 W lamp

Lifespan (hrs) 50 000 50 000 12 000

Company LED Lighting LED Lighting RAD

Colour Cool white and warm white Cool white Cool white

Current 700 mA 350 mA

Luminous flux 8 x 140 = 1120 flux 15 x 90 = 1350 flux 6500 flux

Power @ source 21 W 24,5 W 79,5 W

Table 1: Elements of the test.

Fig. 1: The test venue dimensions.

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down lights and the metal halide down light.At the premises, measurements of the performance of the 8-LED and 15-LED down light and 70 W metal halide down light were observed on a plain white wall, while the tests were done in complete darkness. The test was done at distances of 2,15 m which is the same distance from the ceiling to the measurement plane. The lux levels were measured at standardised points of 0,25 m along the horizontal axis. Different lenses were used on the 8-LED and 15-LED to determine which would most accurately represent the equivalent light levels/output of the 70 W metal halide.Table 3 il lustrates a comparison between, 8-LED and 15-LED down lights, and the 70 W metal halide at a distance of 2,15 m.The above data has been plotted in Fig. 4.Table 4 shows the average light when the fittings are spaced at various distances from each other.Fig. 3 shows the adjacent light from the overlap of the fittings.As seen in Table 4 and Fig. 7, the 15-LED with a 30˚ spread spaced at 1,5 m is the closest comparison to the 70 W metal halide, spaced at 2,5 m. The graph shows that the 15-LED with a 30° lens can achieve the same intensity of light but the metal halide has a much wider spread of light. To achieve the same average of light in a room, the LED fittings will need to be spaced closer together to achieve the same average. The required average lux at a height of 0,85 m above floor level is 600 lux and as shown in the comparison (Fig. 5) the 15 W DL with a 30˚ spread, spaced at 1,75 m, has an average of 622 lux, which is an acceptable lumination. The 24 W DL is not a very good comparison for the 70 W metal halide and does not achieve the required average. The best bet will be to do a cost comparison for all of them.

Cost comparison

The power usage per light fitting is the power drawn at the plug so it includes the power supply and the light source. Based on using more LED down light fittings as in the above light configuration alternatives, the table below shows a cost comparison, taking in to account the following assumptions. Table 6 shows the time for break-even

Fig. 2: The Relux output.

Light fitting and location Location Average lm

150 W sodium Canal walk 33,3

Fluorescent 18 W x 4 Canal walk 194

70 W metal halide Canal walk 510

Fluorescent 4 x 36 W Waterfront 581

70 W metal halide and fluorescent 4 x 36 W Waterfront 698

Table 2: Real life average measurements at floor level.

Fig. 3: Relux 70 W metal halide summary result overview.

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Discussion

The cost comparison indicates that the cost per year for the 24-LED down light, spaced at 1,5 m is expensive compared to the 15-LED down light spaced at 1,75 m. The 15-LED down light spaced at 1,5 m, is R137,75 more expensive than the 15-LED down light spaced at 1,75 m and only achieves 5% more light output. This means that it is more efficient to use the 15-LED down light spaced at 1,75 m with a 30° lens, achieving an average 622 Lux at a height of 0,85 m above floor level, in a room with a 3 m high ceiling.The 15-LED down light needs 42% more fittings in a line because the LEDs are spaced 1,75 m apart and the 70 W metal halides are spaced 2,5 m apart. That is, if one uses 40 x 70 W metal halides in a row, then 57 LED down lights would be required. The 15 LED down lights spaced at 1,5 m need 2,25 more light fittings in the two dimensions than the 70 W metal halides to achieve an average of 600 lux at a height of 0,85 m. At the current electricity price the break-even year is year 6, but Eskom is planning to increase the electricity price

Lux levels 2,15 m away

Distance from centre [m] Metal Halide 8-LED 60 CW 8-LED 45 CW 8-LED 30 CW 15-LED 30 CW 15-LED 60 CW

0 1570 355 500 908 1600 955

0,25 1202 290 327 423 678 720

0,5 693 192 182 227 199 393

0,75 443 142 79 80 104 143

1 208 86 46 41 53 84

1,25 144 53 38 29 33 47

1,5 100 34 36 26 21 32

1,75 98 25 26 19 21 29

Total Lux 4458 1177 1234 1753 2709 2403

Table 3: A comparison between down lights.

Fig. 4: Plots of Lux levels vs. Off-centre distance.

Lux average of adjacent light of overlapping fittings

70 W metal halide spaced 2,5 m 761

8-LED 30˚ spaced 1,5 m 438

15-LED 30˚ spaced 1,5 m 652

15-LED 30˚ spaced 1,75 m 622

15-LED 60˚ spaced 1,5 m 639

Table 4: Average lux with various spacings.

Fig. 5: Comparing the different lamps.

between original cost and value of energy saved for the different light

fittings at the respective spacings.

Fig. 6: An 8 LED down light.

Lux levels vs. Off centre distance

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by 25% each year for the next three years. The increase in the electricity price will usually bring the break-even period down to five years.This study achieves an average of 600 lux, but in many cases an average of 400 lux is acceptable. The 15-LED

Cost comparison LED vs. total metal halide

Assumptions/input info Ref

No of hrs light source used per day A 14

No of days per year light turned on B 365

Electricity cost per kWh C R0,57

LED lifespan in hours D 50 000

LED lifespan in years E 10

Metal halide 70 W lifespan in hours Z 12 000

Maintenance cost to replace globe Y R500

8-LED down light R729

15-LED down light R1100

Metal halide 70 W R860

Table 5: Cost comparison.

Item Ref Formula General lighting

1,5 m spaced 1,5 m spaced 1,75 m spaced 2,5 m spaced

8-LED DL 15-LED DL 15-LED DL Metal halide

Percentage more fittings needed 66% in each direction 66% in each direction 43% in each direction

Initial cost of light source F R2631,60 R3366 R2475 R860

Fitting P Include Include Include Include

Power supply Q Include Include Include Include

Total cost of light source Z F+P+Q R2631,60 R3366 R2475 R860

Power usage per light fitting (W) G 64,26 73,44 54 79,5

No of kWh in lifespan of LED light source H A B E G 3213 3672 2700 3975

Cost of electricity over lifespan of LED light

sourceI H C R1831,41 R2093,04 R1539 R2265,75

No of times MH light replaced over LED light

lifespanJ D/Z 0 0 0 4

Cost of light source during replacement cycle K F+Y R0 R0 R0 R500

Cost of replacement lights during lifespan L J K R0 R0 R0 R2083,33

Total cost over lifespan of LED light source M Z+I+L R4463,01 R5459,04 R4014,00 R5209,08

Total cost/year N M/E R456,12 R557,12 R410,23 R532,37

Fig. 7: Light spread comparison.

Fig. 8: A 15-LED luminaire.

luminaire has a very high peak and can be used where just the merchandise or racks need to be illuminated, and not the floor as well. Many retail stores are already using specific illumination where only the important areas (merchandise) are illuminated with high intensity.

Conclusion

This study has been completed using the best practices in LED technology. This involves using the highest quality LEDs designed to achieve a life expectancy of 50 000 hours. Based on this analysis, it is possible to match the light output of the 70 W metal

halide lamp. However, in order to achieve the equivalent spread, twice the number of 15-LED down lights with 30° lenses would be required. If however the 15-LED down light is used in an application where the fittings are used in a line to illuminate shelving, then only 42% more fittings are required.A recommendation would be to do a real-life mockup installation to provide more proof of the number of 15-LED down lights needed to achieve an equal amount of light.

Contact LED Lighting SA, Tel 021 448-8333, salesct@ledlighting.co.za

Table 6: Lifetime cost comparison.