Multidirectional versus Flat Panel Phototherapy Systems

Performance Comparison:

Multidirectional versus Flat Panel Phototherapy Systems

Solarc E‑Series Expandable versus Solarc 1000‑Series Panel


As discussed in the article Understanding Narrowband UVB Phototherapy, UVB-Narrowband has become the phototherapy treatment of choice for psoriasis, vitiligo, and atopic dermatitis (eczema). This is because, unlike traditional UVB-Broadband bulbs, UVB-Narrowband bulbs (PHILIPS TL‑01) concentrate a very high percentage of their energy at the most medically effective wavelengths in the UVB range (approx. 307 to 313 nm), so the patient can take more therapeutic UVB before reaching the skin burning (erythema) threshold. Unfortunately, this means that UVB-Narrowband doses are larger, which can be achieved only by having longer treatment times or using a device that has more bulbs and/or is more efficient at delivering the light to the patient’s body. Modern UVB-Narrowband devices, therefore, benefit greatly from efforts to improve device efficiency.


Treatment Time: The amount of time in seconds given to the patient per treatment position, (such as front-side, back-side, left-side, right-side).

Irradiance: Light power per unit area; in UVB-Narrowband phototherapy, usually with the units of milliwatts per square centimeter (mW/cm^2). Irradiance is sometimes also called Flux (flow per unit area), or perhaps “intensity” (a brighter light fixture has more irradiance, for example as do the Solarc 1000‑Series devices with more bulbs).

Dose: Light energy per unit area, in UVB-Narrowband phototherapy, usually having the units of millijoules per square centimeter (mJ/cm^2). The maximum dose is limited by the patient’s tendency to burn.

The above three variables are governed by the equation:

TIME(s)= DOSE(mJ/cm2) / IRRADIANCE(mW/cm2)

So to deliver a certain Dose, a device with greater Irradiance will have a shorter treatment Time. 

Delivered Power: Is relative value devised by Solarc to represent the total amount of UVB-Narrowband light power delivered to the patient’s body at any single treatment position setup. It is the integrated sum of the irradiance value at a measurement point multiplied by the area element irradiated (calculus). Or put another way, because the irradiance varies around the patient’s body, it is each measured irradiance value multiplied by the area that the irradiance value covers, and then all added-up. The device designer’s goal is to maximize Delivered Power, which can be achieved by using more bulbs and/or improving the efficiency of delivering the light produced by the device to the curved skin surfaces of the patient.

Session Time: Is the sum of all the treatment times in one patient phototherapy session. For example, if the patient gives 1 minute each to their front-side, back-side, left-side, right side; the Session Time is 4 minutes. Session Time represents the total amount of “lights-on” time the patient spends getting their UV light treatment, and is the value that we are most interested in minimizing. Session Time is inversely proportional to Delivered Power (a device with greater Delivered Power will result in a shorter Session Time). Devices that wrap around the patient’s body, such as the Solarc E‑Series, can have a lower Session Time because fewer patient positions are needed to provide full coverage around the patient’s body.


The purpose of this study is to compare the performance of two different 6-foot Full Body UVB-Narrowband phototherapy device types:
A)  A traditional “Flat-Panel” (a single device with bulbs arranged side-by-side in a line) and…
B)  A “Multidirectional” system (multiple interconnected devices that are arranged at a different angle to each other).
In particular, for each device type, this study attempts to:
1.   Measure the Irradiance pattern of UVB-Narrowband light delivered to an idealized patient body shape. How much better are multidirectional devices at evenly distributing the light around the patient’s body?
2.   Using the results of 1., estimate the Delivered Power on the idealized patient body shape, which is inversely proportional to the Session Time. We want to maximize Delivered Power so the Session Time is minimized. How much more light energy can multidirectional devices deliver by being positioned around the patient’s body?
3.   Quantify the cost effectiveness of the two device types. Complete this by dividing the device cost by the Delivered Power. Are multidirectional devices more or less cost effective than flat-panels? Can a multidirectional device use fewer bulbs than a flat-panel device, yet have the same Session Time?

Devices Tested:

The two device types tested were:
1. Multidirectional: The Solarc E‑Series in five(5) assembly configurations: 1M (Master Device by itself, 2-bulbs) 1M + 1A (4-bulbs) 1M + 2A (6-bulbs) 1M + 3A (8 bulbs) 1M + 4A (10 bulbs)
2. Flat Panel:  The Solarc 1000‑Series ; model 1790UVB‑NB (10-bulbs). Thousands of 1000‑Series devices have been successfully used since first introduced in 1992. These devices are excellent candidates for comparison because the devices have the same minimum treatment distance of 8 to 12 inches; and the same bulbs, lampholders, ballasts and general construction methods. The anodized aluminum reflector material is also the same, but the E‑Series likely has some advantage from angled reflector facets for each bulb, versus the 1000‑Series flat reflector with parabolic sections at the outermost bulbs. The same set of 10 bulbs (PHILIPS TL100W/01‑FS72 datecode:0G) were used for all tests.


Patient body shapes vary considerably, from thin to obese, so a simple vertical cylinder was selected as the idealized patient body shape. Two cylinders were considered; one with a diameter of 12 inches (about a 38-inch waist), and another with a diameter of 24 inches (about a 75-inch waist). To measure the irradiance on the surface of the theoretical cylinder, a jig was made to hold the UVB-Narrowband light meter sensor on the end of a variable-length arm that could be rotated about 100° to each side of the device centreline, and positioned by the protractor on the base, as shown here.

To make the irradiance measurements, the light meter sensor window was located 10 inches from the front face of the bulbs; midpoint of the minimum treatment distance range of 8 to 12 inches. In the case of the E‑Series, all devices in the assembly were positioned to maintain the 10 inch distance, forming a curve to surround the theoretical cylinder defined by the path of the light meter sensor. The devices were operated until the irradiance reached steady-state (about 30 minutes), and irradiance measurements taken every 5°, using the protractor under the arm to set the angle. Voltage and frequency were exactly the same for all tests, maintained by a high-capacity electronic power supply. The ambient temperature for all tests was practically the same.

multidirectional phototherapy performance test jig1 Solarc Performance Comparison
multidirectional phototherapy performance test1 Solarc Performance Comparison

To estimate Delivered Power, the irradiance graphs produced were radially integrated (the UVB-Narrowband irradiance value at each 5° angle summed together – which is also the area under the curve) and multiplied by the arc length, which the 5° segment creates at that patient diameter. Then, for easier comparison, the Delivered Power values were “normalized” by multiplying each of them by the one constant number that makes the 1790UVB‑NB have a value of 1.0. Normalized values are comparable only to those of the same idealized patient body diameter. Where necessary and reasonable, some irradiance curves had to be extrapolated on their ends to make the integrations more complete.


The radial irradiance measurements are shown in the following two graphs, one each for the 12-inch cylinder and 24-inch cylinder. The results of the integrations of the curves in the graphs (to estimate the Delivered Power of each device) are in Table 1 after the graphs.

multidirectional phototherapy performance test 121 Solarc Performance Comparison
multidirectional phototherapy performance test 241 Solarc Performance Comparison
Table 1 – Total Relative Energy Results  (Irradiance Curve Integrations)
Device TypeAssembly ConfigurationNumber of BulbsDelivered Power to a 12-inch dia. Idealized Patient [normalized to 59.6]Delivered Power to a 24-inch dia. Idealized Patient [normalized to 95.7]
Flat Panel Solarc 1790UVB‑NBSingle Device1059.6 [1.0]95.7 [1.0]
Multidirectional (1) Solarc E‑Series E7201M221.2 [0.36]29.9 [0.31]
Multidirectional (2) Solarc E‑Series E7201M+1A442.0 [0.70]57.8 [0.60]
Multidirectional (3) Solarc E‑Series E7201M+2A666.1 [1.11]90.6 [0.95]
Multidirectional (4) Solarc E‑Series E7201M+3A8incomplete data*124.1 [1.30]
Multidirectional (5) Solarc E‑Series E7201M+4A10incomplete data*160.3 [1.67]

* The 12-inch cylinder integrations for assembly configurations 1M+3A and 1M+4A were not made because the light meter jig did not allow full rotation to the higher angles, causing the irradiance curves to be incomplete and not extrapolated with much confidence. However, using a simple linear projection, we expect the 12″ Normalized Total Relative Energies to be about 1.5 for 1M+3A, and about 1.9 for 1M+4A.

Conclusions and Discussion:

To answer question 1 posed in the Purpose, the graphs show that a multidirectional device indeed has more even UV-light distribution, best seen by comparing the large flat section in the E‑Series 1M+4A irradiance curve to that of the strong peak of the 1790UVB‑NB. Even the E‑Series 2-device 1M+1A assembly configuration shows benefit from angling the devices, as evidenced by the relative flatness of its curve. A flatter curve represents more uniform light distribution, which reduces the chances of localized overexposure and/or the need for the patient to reposition frequently to average-out the dose.

To answer question 2 posed in the Purpose, Table 1 shows that a multidirectional device can indeed deliver more total UV-light to the patient’s body, and nearly regardless of patient body diameter. It is observed that the 3-device E‑Series configuration (1M+2A) with just 6 bulbs delivers about the same total amount of UVB-Narrowband light as a 10-bulb 1790UVB‑NB flat panel, so the two devices would be expected to have about the same total treatment time. Table 1 also suggests that a 10-bulb E-Series setup can deliver 60 to 90 % more UVB-Narrowband light than a 10-bulb flat-panel. 

To answer question 3 posed in the Purpose, it has been shown that a 3-device E‑Series 6-bulb setup costing about $3200 is about equal in performance to a $2900 Solarc 1000‑Series 10-bulb model 1790UVB‑NB. Regrettably, we did not test lesser 1000‑Series models with fewer bulbs, but we certainly expect that the 2-device E-Series 1M+1A 4-bulb setup costing about $2200 to outperform to the $2000 Solarc 1000‑Series 4-bulb model 1740UVB‑NB (which we will probably discontinue), and possibly even the $2300 Solarc 1000‑Series 6-bulb model 1760UVB‑NB. The other features of the E‑Series, such as expandability and portability, are intangibles with value known only to the purchaser.

The ability of the $1200 E‑Series Master device to be used by itself is also obviously a great option for those on a limited budget, or those uncertain if UVB-Narrowband phototherapy will be effective for them.

Finally, the E-Series’ ability to expand and provide performance well beyond that of a 10-bulb flat-panel gives all users the option of eventually building a very high performance system. For example, on a single 120-volt, 15-amp circuit, the 10-bulb E-Series 1M+4A configuration can deliver 60 to 90 % more UVB-Narrowband light than the 10-bulb 1790UVB‑NB flat-panel.

For these reasons, we believe that the E‑Series is a superior product, but we also believe that the 1000‑Series will likely always have a role; it is hard to argue with 20+ years of success, and 1000‑Series devices do have a lower cost-per-bulb.

Other Observations:

1. For the E‑Series, and due to overlapping of the light delivered from the devices, we see that the maximum irradiance values (the peaks) increase for each device added, up to 3 or more devices where the maximum irradiance stabilizes. The E‑Series User’s Manual therefore provides different exposure guideline tables and treatment times for 1-device, 2-devices, and 3-or-more devices.

2. The Delivered Power values of the 12-inch and 24-inch diameter tests can be compared to each other because the integrations were multiplied by a factor (the 5° segment arc length) that represents the area exposed to the light. The 24-inch diameter tests show greater Delivered Power values than the 12-inch diameter tests, which we believe to be because the target is simply larger. A smaller diameter target improves the relative performance of the multidirectional E‑Series (as evidenced by the 12-inch test’s greater normalized values), because it can better wrap around the surfaces being treated.

3. The E‑Series Delivered Power values increase nearly linearly with the number of devices, which is as expected and evidence of a successful experiment.

4. Because an actual cylinder was not mounted in the devices for the tests, we suspect that there is a small amount of cross-reflected light that is boosting some irradiance curves, in particular those for the 12-inch diameter tests, and especially for 1M+4A as its two outermost devices point almost directly at each other. If an actual cylinder was mounted into the devices to mimic a human body, it would block some of this cross-reflected light.

Bruce Elliott

P. Eng.