On Target Near Infrared Tutorial
Revised March 2010
What is Near Infrared?
The electromagnetic spectrum is simply a plotted distribution of all radiant energies as a function of their wavelength. It ranges from the shorter wavelengths of x-rays and gamma rays to the longer wavelengths of radio waves and microwaves. Remember, wavelengths and frequencies have an inverse relationship; high frequency means shorter wavelengths and visa versa. There are several regions of the electromagnetic spectrum that are useful for remote sensing. The most widely used for vegetation analysis is near-infrared (NIR).
NIR is a small portion of the much larger region called infrared (IR), located between the visible and microwave portions of the electromagnetic spectrum. NIR makes up the part of IR closest in wavelength to visible light and occupies the wavelengths between about 700 nanometers and 1500 nanometers (0.7 µm - 1.5 µm). NIR is not to be confused with thermal infrared, which is on the extreme other end of the infrared spectrum and measures radiant (emitted) heat.
What Does NIR Tell US?
Since NIR has longer wavelengths than visible light, it exhibits peculiar properties that can be exploited for remote sensing applications. Some of the information that can be obtained from NIR is crop stress (water and nutrient stress being the most common) and weed/pest infestations.
Chlorophyll pigment absorbs most energy at about 650 nm (red) and around 450 nm (blue). Other pigments absorb more visible wavelengths, but the most absorption occurs in the red and blue portions of the spectrum. This absorption removes these colors from the amount of light that is transmitted and reflected, causing the predominant visible color that reaches our eyes as green. This is the reason healthy vegetation appears as a dark green. Unhealthy vegetation, on the other hand, will have less chlorophyll and thus will appear brighter (visibly) since less is absorbed and more is reflected to our eyes. This increase in red reflectance along with the green is what causes a general yellow appearance of unhealthy plants.
Another feature of vegetation is the strong reflectance within the NIR. Since NIR is not absorbed by any pigments within a plant, it travels through most of the leaf and interacts with the spongy mesophyll cells. This interaction causes about half of the energy to be reflected and the other half to be transmitted through the leaf. In plants with turged and healthy mesophyll cell walls and in dense canopies, more NIR energy will be reflected and less transmitted. This cell wall/air space interaction within these cells causes healthy vegetation to look very bright in the NIR. In fact, much more NIR is reflected than visible.
By monitoring the amount of NIR and visible energy reflected from the plant, it is possible to determine the health of the plant.
High NIR reflectance / Low visible reflectance = Healthy
Low NIR reflectance / High visible reflectance = Unhealthy
Because of the wider range of reflectance between healthy and unhealthy vegetation within the NIR region, sensors that detect within this region are much more sensitive to subtle changes in plant health. These sensors use silicon based arrays to measure the amount of NIR and visible energy reflected. These devices will often convert the result into one of several indicies such as the normalized difference vegetation index (NDVI). In this way, the health of vegetation can be constantly monitored.
The Digital Camera
Digital cameras use the same silicon based sensors that are used for remote sensing. The CCD (charge-coupled device) and CMOS sensors found in digital cameras are quite sensitive to NIR. This would make these consumer cameras ideal for monitoring vegetation. However, since these cameras are sensitive to IR, digital camera manufacturers have installed special internal IR cut filters (hot filters) designed to reduce IR contamination in visible light photos. This, of course, is done since most people want to see visible light in their images and not IR. These filters vary in the amount of IR that they transmit (since no filter blocks 100% of anything) and therefore some cameras will work much better for NIR imaging than others.
In order to use a digital camera for NIR work, a special external filter is needed to filter out all visible light and allow only IR to pass through. These filters are known as IR pass filters and have been used for years by film-based IR photographers and are available at many camera store locations.
Since digital cameras use no film and all image information is recorded at the CCD, they provide many advantages over film-based IR photography:
- Instant images
- Proper exposures since it uses its CCD to measure light and set the shutter speed and aperture
- Affordable - no more expensive IR film
Testing for IR Sensitivity
Not all digital cameras are capable of IR photography. Before purchasing an IR filter for a camera, make sure the digital camera will even work. The hot filters installed on cameras vary greatly so it is best to check the camera to be sure it will be sensitive enough to NIR to capture quality images.
Here is a simple test that can test the sensitivity of a digital camera to NIR:
- Aim an infrared remote control (like the one you have for your television) at the camera.
- Press any button on the remote.
- If you can see a bright light similar to the one at right emitting from the front of the remote control through the camera, then the camera is sufficiently sensitive to the NIR band to use.
IR Filter Summary
Because a digital camera's sensitivity to infrared is marginal when compared to visible light, even a slight difference in transmittance may have strong effects on results. Therefore, a filter that works on one camera may not work at all on another camera. If you can, be sure to test the filter with the camera before purchasing the filter.
The filters that we have used and had the most success with are the Hoya R72 and RM90. Both are good filters, though the RM90 is better suited for crop stress detection since it blocks all red light and therefore gives a truer indication of the amount of IR reflectance. However, it also may not work on many cameras since it blocks about 50% at 900nm. You will need a very sensitive camera to use this filter. The R72, on the other hand, can be used on a wide range of cameras and does a fine job.
"I Can't Find a Filter That Will Fit My Camera"
There are about as many filter thread sizes as there are cameras. You may find it difficult to obtain a filter that fits your particular camera, or you may have a filter that fits a different camera but does not fit the one you want to use. In this case, you may use a step-up (or step-down) ring to accommodate the different thread sizes.
This is particularly true with the Nikon 900 series of cameras that use the small 28mm thread size. A step up ring is necessary to use these series of cameras since no IR filter is made for that size.
Filters are expensive. By using step up rings like the ones shown, you can extend the use of your filters.
The following are some common thread sizes for cameras that we have used:
- Nikon 950/990 28mm
- Sony MiniDV camcorder 37mm
- Canon DV camcorder 30.5mm
- Sony Cybershot 5.0 MP (DSC-F707) 58mm
These cameras are also good with NIR photography, although they are far from the only ones that will work.
The NIR images that you receive from a digital camera are monochrome, meaning they appear like black and white photos. It is possible to make the image look like an 'Ektachrome' color IR photo by combining a standard RGB photo with the NIR image in Adobe Photoshop.
Taking both images can easily be done with a tripod and a single camera where one image is taken with the filter and the other without.
One method we are testing is using two digital cameras in tandem, attached to a helium filled blimp. These cameras are then operated from the ground by a serial connection and images are stored on flash memory cards.
The image on the left is the test balloon (blimp) used to lift two Nikon CoolPix 950 cameras above test plots at Utah State University (notice the cameras hanging below the blimp). The image on the right is the combined NIR/Color image taken from 250 ft. This image was taken later in the year after much of the wheat had already ripened (shown by the lack of red).
By using consumer digital cameras to obtain IR images, a farmer could detect crop stress early and receive images instantly at a fraction of the cost of other methods.