Introduction: Low Cost Bili-Light Radiometer

designed by Greg Nusz and Advait Kotecha

The objective of this instructable is the production of a low-cost, easy to use, low-maintenance device for measuring the efficacy of phototherapy lights bili-lights for the treatment of hyperbilirubinemia (jaundice). The purpose of this device is to measure the output of phototherapy units and to ensure that the emitted light is intense enough (>4uW/cm2/nm) within the correct wavelength range (425 - 475nm).

The device operates by filtering the incident light through blue-glass filters. The light that passes through the filters is then collected by a solar cell where it generates a current that is read as the device output via an onboard ammeter. Because the measured current is generated by the incident light, no other power source is needed.

Directions for Use:
The meter should be held the same distance and direction from the bililight as the baby receiving treatment. The needle indicator in the red indicates that insufficient light is emitted by the light in the wavelength range 425-475nm, and the bulbs should be replaced. The needle indicator in the green indicates that there is sufficient blue light in the therapeutic window to treat hyperbilirubinemia.

Limitations
The primary limitation for this device is related to the filter inability to completely block infrared (IR) light. Since silicon has a high responsivity even the 5% that passes through the filter can contribute to the signal and thus cause false positive readings in the presence of IR. For this reason, the radiometer will not provide accurate readings for incandescent bulbs are outdoors. However, most bililights in use are either fluorescent or LED-based.

The documents attached are a word document version of this instructable as well as a instruction sheet in doc and pdf format.

This device was developed in cooperation with Engineering World Health. For more info on EWH, visit their website http://ewh.org/

Step 1: Parts List

34mm Blue Colored Glass Filter x 2
Pegasus Associates Lighting PCGF-MR11-BLU 2 @ $5.90 = $11.80

0-1mA DC Ammeter
Marlin P. Jones & Assoc. 8726 ME $13.95

Solar Cell 0.5V, 300mA
Edmund Scientific Item # 3081612 $6.95

Project Box: HAMMOND Multipurpose Instrument Enclosure 4.72 in x 3.15 in x 2.17 in
Newark Electronics, Newark Part Number: 87F2528, Manufacturer Part No: 1591TSBK
$5.84

Fasteners $0.20

Total $38.74

Step 2: Filter Description

Filters- The blue glass filters from Pegasus Associates Lighting were selected because their transmission spectrum closely matches the absorption spectrum of bilirubin. Two were used to further diminish transmission of non-therapeutic light. Also, the 34mm round filters fit nicely with the solar cell selected. It would be possible to buy blue glass in bulk from an art supply company and cut the pieces necessary for use, although the transmission spectrum of the glass should be measured first. The diagram shows the transmission spectrum of the dual-filter setup with the absorption spectrum of bilirubin overlaid.

Step 3: Remaining Parts Description

Ammeter We selected the 0 -1 mA DC ammeter from MPJ because it offered high
precision measurements (+/- 2.5%) for the low currents generated by the solar cell.

Solar Cell Edmund Scientific offers several models of solar cell. The model we selected was chosen because of its relatively high current output for its size, the fact that the leads are already connected, and because of the casing that includes plastic lensing that allows for more efficient collection of light.

Project Box The dimensions of the box selected are purely a function of the size of the solar cell and the ammeter.

Fasteners: The only fasteners required for full assembly of the device are three nuts and three bolts (~1/8in diameter and at least 3/4in long).

Step 4: Cut Large Holes

1. For the ammeter, cut a round 2-3/8in diameter centered on front.
2. Two 1/8in holes for ammeter mounting screws 1-3/4in from center of the large hole and 2-17/32in from each other (See picture).
3. Filter hole 1 3/4in diameter centered on the top.

Step 5: Drill Holes for Mounting Screws

Drill three holes (~1/8in depending on bolts used) on the top so that the edges of the new holes are 1/8in from the edge of the filter hole (See picture). These holes are for bolts that should just touch the edge of the filter (see picture).

Step 6: Drill Holes Through Solar Cell

Clamp solar cell to the box with the cell facing up out of the filter hole (See picture) and re-drill the same holes for the mounting holes through the casing of the cell. Be sure that the cell is in far enough so that the back will close. Also be careful not to damage the cell or wiring while drilling! It may also be necessary to remove any plastic bits from inside the cell casing if they are not drawn out while drilling.

Step 7: Insert Mounting Screws

Insert the mounting screws, through both the box and the solar cell so the heads are on the outside of the box. Put nuts on the bolts just below the cell, but do not tighten them.

Step 8: Insert Filters

Wipe the surface of the filters with a dry rag to remove any fingerprints, especially those surfaces that will be inaccessible after mounting. Insert the filters between the cell and the box and tighten the nuts. Be careful not to crack the cell casing.

Step 9: Install Ammeter

Install ammeter by putting it through the holes in the box and attaching the two mounting nuts. Also connect the leads from the solar cell to the ammeter, attaching the black wire from the cell to the negative pole of the ammeter marked with a negative symbol. Close the box by screwing on the back panel.

Step 10: Calibrate Bili-meter

For ease of use, we use the following ring image to provide a yes/no response from the radiometer. The idea is to put the green/red interface at the current level where enough blue light is available to be phototherapeutic (4 uW/cm2/nm). Thus, the ammeter needle will read in the green for currents higher than the calibration current and red for currents generated that are below the threshold calibration current. This current will vary a bit from device to device and is best determined for each unit independently. Obviously, this requires some additional hardware. The units described here were calibrated using an Olympus Bili-meter. For the three units tested, calibration currents found were 0.12 mA, 0.18 mA and 0.14 mA. Any change in construction or components will alter this calibration current and as such, any radiometers modified from these directions must be independently calibrated.

Step 11: Directions and Limitations for Use

Directions for Use:
The meter should be held the same distance and direction from the bililight as the baby receiving treatment. The needle indicator in the red indicates that insufficient light is emitted by the light in the wavelength range 425-475nm, and the bulbs should be replaced. The needle indicator in the green indicates that there is sufficient blue light in the therapeutic window to treat hyperbilirubinemia.

Limitations
The primary limitation for this device is related to the filter inability to completely block infrared (IR) light. Since silicon has a high responsivity even the 5% that passes through the filter can contribute to the signal and thus cause false positive readings in the presence of IR. For this reason, the radiometer will not provide accurate readings for incandescent bulbs are outdoors. However, most bililights in use are either fluorescent or LED-based.

Comments

author
GorillazMiko (author)2007-12-20

looks a little complicated, but looks cheap too!

author
Normjr (author)GorillazMiko2009-09-05

I must disagree! I believe that this is a professionally constructed and well written Instruction! It appears to be Inexpensive, NOT CHEEP, like all of those mint box constructs. Kudos to you GJNusz! What might the uses of this project be in the non-clinical world?

author
jongscx (author)GorillazMiko2007-12-21

I actually beg to differ. It's actually pretty simple, just an ammeter hooked up to a solar cell, and some blue filters in front... It is; however, really detailed. How do you calbrate it though? I'm assuming you'd need to get a baseline at the hospital/med equipment store first.

author
GJNusz (author)jongscx2007-12-25

Calibration was performed by using a commercially available Olympus Bilimeter (~USD$2000). Samples were taken at various bililight intensities and we plotted the ammeter output versus the Olympus reading. The ammeter response response was linear throughout a large range of light intensities(R > 0.99). The calibration current was then determined by where this line crossed the minimum therapeutic intensity of 4uW/cm2/nm.

The three prototype units I've made have calibration currents at 0.12 mA, 0.18 mA and 0.14 mA. Ideally, each unit would be calibrated independently by some device like the Olympus Bilimeter as jongsx mentions.

Although, perhaps as more are built we can collect some statistics on calibration currents and see how much it varies from device to device.