SIGNAL DECREASE DUE TO THE PRESENCE OF THE LONG PATH CELL
When a long path cell is placed in a spectrometer sample compartment, there will be loss of energy and disturbance of the infrared beam. These things produce signal reduction, with corresponding loss of measurement sensitivity.
There are three places where the signal loss originates: (1) at the input side of the cell, (2) within the cell, and (3) at the output side of the cell.
Energy Loss at the Input
On the input side of the cell, energy may be lost at the entrance aperture and at the first collecting mirror. Energy loss at the entrance aperture may come about if the area of the infrared image is larger than the allowed image area of the cell. Fortunately, entrance aperture loss is not usually significant because the infrared image is usually smaller than the cell’s image area.
The more likely place for input side energy loss is at the first collecting mirror. This is especially the case for cells with a long base path. In such cells the spread of the beam simply causes much of the energy to miss the first collecting mirror. Sometimes this is called f-number mis-match.
f-number mis-match may be corrected with a lens or mirror that narrows the beam-spread. The energy needs to be directed through an enlarged infrared image. The new image cannot be too large, however, because of the entrance aperture limitation. In some cases, it is necessary to consider a trade-off between image area and beam spread. This is illustrated in the figure here.
Energy Loss Within the Cell
The components of the cell reduce the energy in two ways: (1) by reflections at the windows, and (2) by absorption at the mirror surfaces.
The reflections at the four surfaces of two potassium chloride windows will remove about 15% of the energy from the beam. (about 4% per surface). The reflections at the four surfaces of two zinc selenide windows will remove more than half of the energy.
The amount of energy removed by absorption at the mirror surfaces depends on the reflection coefficient of the mirror coating and on the number of reflections within the cell. A single reflection from a good coating of protected silver can conserve 99.5% of incident infrared energy. The other 0.5% disappears mainly by absorption. This allows many passes through a cell without significant reduction of the beam intensity. A coating of protected gold might reflect 98.5% of incident infrared energy, with 1.5% being absorbed. This limits the number of allowed passes within a cell.
When mirror surfaces become soiled or corroded, the amount of absorption increases, and loss by scattering also sets in.
In figure 2 we present a nomogram of signal reduction as a function of reflection coefficient and number of reflections.
Energy Loss at the Output Side of a Cell
On the output side of a cell, energy is not lost, but it can be wasted because it is not properly focused on the sensing element. In many cases, the spoiling of the focus is the largest component of the signal reduction.
The detector focusing mirror has to concentrate onto the sensing element a reduced image of the focal area. If the cell has moved the focal area to a different distance from the detector focusing mirror, the focus at the sensing element becomes a blur. Energy then misses the sensing element, and the signal is reduced.
One must also consider the optical center-line. This line is re-directed by the transfer mirrors of the cell, but it still exists, and the cell’s focal area must be on that line. If the cell has moved the focal area away from the center-line, the focused energy may be moved off to the side of the sensing element. The signal may then fall almost to zero.
REMEDIES FOR THE THREE SOURCES OF ENERGY LOSS
Energy Loss at the Input
1. If the beam spread is too great for the cell, reduce it with an external focusing mirror.
2. If the beam is nearly collimated, use an external focusing mirror to make an appropriately-sized focus at the input aperture.
3. With a laser in place, execute the adjustments of the input transfer mirror in order to put the laser beam in at the correct location just off the edge of the field mirror and into the center of the first collecting mirror. Observe the arrays of spots, and make sure that the laser beam is properly going out through the exit aperture. With a variable-path cell, this may involve adjustments of the cell’s external mirror controls.
Energy Loss Within the Cell
1. Use clean windows that have a relatively low index of refraction. In the experience of Infrared Analysis, Inc., the best choice of window material is potassium chloride.. Avoid using zinc selenide and KRS-5.
2. If your cell is to be used at 60 passes or more, call for protected silver mirrors. Otherwise, call for protected gold. Keep the mirrors clean.
Energy Loss at the Output Side of the Cell
1. While observing the signal, execute the adjustments allowed by the exit side transfer mirror, including the lateral movement of the mirror mount. If you can see a laser beam coming out of the cell and you can see the sensing element, make sure the red light is properly focused on the sensing element.
2. If possible, move the cell in the sample compartment slightly right-to-left, up-and-down, and front-to-back, while observing the size of the detector signal.
3. If you have access to the detector focusing mirror, adjust it to make sure that the focus is sharp and is exactly on the sensing element
NOTE: This last remedy is probably the most important remedy of all. Every cell user should make an effort to obtain a perfect focus on the detector element..