It has long been recognized as Nikon DSLRs are boosted by Sony sensors since D100. Now in the 3rd generation cameras, Nikon switch to Sony Exmor CMOS sensor from HAD CCDs. These CMOS sensor utilized on-chip column parallel Analog-to-Digital Converters (ADC) running at very low clock rate around 20kHz compared to serial external ADC that runs at around 10MHz (Such as D3/D700/D3s and Canon DSLRs. Calculation as follows: 12.2MP x 9FPS / 12 Channel = 9.2MP/ (Second x Channel) ). The slower clock rate and integration of ADC on chip significantly reduces read noise, as it would be beneficial for dynamic range.
The latest of all, D7000, has been analyzed by Chipworks and they had found a Sony IMX071 Exmor sensor within. Test done by Dxomark indicated increased sensitivity compared to previous CMOS sensor (D90 and D300s). Interestingly, either Nikon nor Sony advertise their gapless microlens structure as Canon always did, but it has been clearly revealed that IMX071 microlens array does not have gaps in between. (Fig 1)
Figure 1. Chipworks teardown shown pixel vertical structure. Above: IMX071 gapless microlens. Below: Canon 5D and Canon 5D Mark II
Combination of significantly improved Quantum Efficiency (QE), lowered read noise and dark current at room temperature against CCDs, these DSLRs should be highly competent in astrophotography. However, one more obstacle remains: DSLR are not sensitive to hydrogen alpha line (656nm) from most of the emission nebulas simply because camera manufacture add on a color correction filter to mimic the response of human eye. This color correction filter absorb a large proportion of red and almost all infrared photons while leaving green and blue channel untouched. The silicon based sensor itself are highly sensitive in the entire visible spectrum ranging from 400 to 700nm, and extending towards near infrared of 1100nm with much lower response. So the simple answer to this is remove it!
Warning!
Please use this instruction at your own risk! I will not be responsible if you accidentally break your camera in the process! If you have shaky hands and are not confident with doing this yourself, send it to a qualified company for conversion or don’t do it!
Filter Structure
But removing it cause another problem when large amount of infrared rushes towards the sensor, then how do we solve it? So let’s take a look the filter in detail.
Actually the filter is more complicated, it consists of 2 parts: a dust reduction filter, and a sandwich of 3 layers of glass stitched together. The later is usually called ICF (infrared cut filter), or sometimes by its other property – Anti-aliasing filter, or optical low pass filter (OLPF). Each layer has its own function. This filter stack not only serve to suppress infrared leakage and correction of color in red channel, but blurs the image on pixel level to reduce the color moiré inherited in Bayer sensor. The dust reduction filter and the last layer of ICF are both similar in thickness and are made of birefringent material. And the first layer of ICF is a wave plate similar to the one in your circular polarizing filter. When oriented 90° to each other in conjunction with the wave plate, they separated the light in horizontal and vertical direction respectively, translating one dot into 4 and thus blur the image. The thickness of both are tuned according to the pixel pitch. As a general rule, the bigger the pixel, more blur you need and thicker the birefringent layers.
D7000 sensor module showing inner filter stack on top and a dust reduction filter disassembled on right.
The middle layer in the ICF is the color correction filter that absorb deep red photons and infrared. Another thing to notice is the dust reduction glass is coated with an interference filter that blocks UV and IR. The sharp cutoff allows greater than 95% of visible spectrum between 430 to 680nm to pass through. So in this modification, we will only remove the 3 layers ICF stack while preserving the dust reduction filter that blocks UV/IR as well as keeping dust away. Removing this filter also change the infinity focus distance and you may not be able to achieve focus at infinity for some lenses. So I ordered a clear piece of optical glass with 1mm thick just in case.
Modification
The detailed disassembly step can be found at Lifepixel website: http://www.lifepixel.com/tutorials/infrared-diy-tutorials/nikon-d7000-ir
Before tearing down your camera, prepare the small boxes for preserving the screws. I used the petri dish to help me memorize the same batch of screws from the same panel. Duct tape can also help you organize the screws. Also be sure to ground yourself and not to do it during cold and dry winter to prevent static charge from damaging the delicate electronics inside. The screwdriver you need is #000 or 5/64" Phillips type. Do not use flash before conversion nor open the flash. This will charge the capacitor and may give you an electrical shock once you touch it, or destroy the circuit when you accidentally short circuit inside.
The disassembly step are as follows:
1. Remove battery, eyepiece rubber cap and SD cards. Remove the lens and cap the body.
2. Unscrew and remove the bottom panel
Bottom panel removed and it is made of plastic
3. Unscrew the ones on SD slot compartment, viewfinder, one on the left and 3 silver ones in the bottom. Gently lift LCD panel unit.
The back panel is made of magnesium alloy.
4. Carefully flip the hinge connector using your finger nail or a forceps and gently release the ribbon cable from the slot. Be sure to identify which part is the rotating bar and which fixed jack. This is the most crucial and tricky step and you need to be really careful when dealing with these flat cables. Make sure all the cable are properly released before the next step.
The main PCB with Toshiba microcontroller chip. Ribbon cable are disconnected.
5. Unscrew the 6 silver screws on the PCB. Lift the PCB from right side and slowly pull the PCB to the right since the USB/HDMI jack is inserted into the plastic panel on the left of the camera.
The back side of PCB, where the virtual horizon sensor is in the bottom right corner. Removing the PCB exposes the sensor frame. The sensor power supply cable is L shape and the sensor output in 8 LVDS channels (+1 for Clock) to the main PCB.
6. Now it’s time to access the sensor. From this step on, you should consider dust and your dirty hands very seriously. I’m converting in a laboratory fume hood and wear latex gloves from this step onwards. The 3 big silver screw in the above image fasten the sensor module to the fore body of the camera. Once you tweak it, the focus calibration is no longer valid. To proceed, release the last ribbon cable and unscrew the sensor board. I suggest you mark the screws with color marker pen to distinguish the screw with its mounting hole, as well as its preset rotation position.
Mark the screw before proceed. The CMOS sensor is driven by a 54MHz crystal oscillator.
Now move to the hood
The sensor module and the shutter blades
7. The piezoelectric elements attached to the dust filter is connected via ribbon cable as well. But we do not need to desolder it. Unscrew the 4 holding the dust filter to the sensor unit and gently lift the dust filter frame (2 more on the right side in hidden below the aluminum foil). Once the screws come loose, gently lift the dust filter. Use the forceps to lift one corner of ICF and take it out. Reverse the steps and assemble your camera.
After removal, sensor became much more transparent
ICF Structure
The dimension of stack is 29.50mm x 25.30mm measured with a vernier scale. The wave plate and color correction filter is a little bit smaller, giving a 21.50mm in height. This dimension has been standard for Nikon since D70. The thickness reading from a micrometer yields a 1.182±0.002mm for the entire stack and 0.538±0.003mm for the transparent anti-aliasing glass. This means the color correction filter and wave plate is 0.64mm thick, which is identical to that in D90.
The cross section of ICF stack under microscope, showing 3 layers of wave plate, color correction layer and birefringent glass from bottom to top. The top is facing the CMOS sensor.
Focus Shift
Since we did not replace the ICF stack with a clear sheet of glass of equivalent optical depth, the camera now will become significantly nearsighted, and the phase detection focus will be useless. In my test, I set both lens and default focus fine tune to +20. The result is still slightly blurred. And for lenses with hard infinity focus stop, it will become impossible to achieve sharp focus at distance. Right now the only lens I have that can obtain focus at infinity is AF 180 2.8D, for this lens has to have a large tolerance of infinity for its focus shift of ED elements dependent on temperature variation.
The Liveview infinity manual focus position largely deviates from original infinity focus, where bar should point right in the middle of the "
"
My 18-105 DX seems to achieve infinity focus at zoom range greater than 30mm but not the wide angle end.
Sharpness Test
The removal of of ICF along with its anti-aliasing filter should increase the resolution on the pixel scale. This is in principle similar to recently announced D800E. However, as we had preserved the first AA layer for its IR rejection coating and dust reduction, this camera will have astigmatic vision. The images below are shot at ISO12233 chart at 70mm and fixed distance using liveview focus. NEF raw is taken and demosaic using MaxIM DL. These section are cropped at 200% from the same image.
After conversion, the vertical resolution is increased compared to horizontal. And the horizontal resolution remained the same (See below)
Before conversion, resolution is the same on 2 directions.
This test suggests that the dust filter separates the image on horizontal direction and the last layer of ICF blurs the vertical one.
OLPF under microscope
To investigate under a microscope, the last transparent layer of ICF stack is place between a hemocytometer and 40x objective. The image below clearly illustrate the effect of the OLPF layer with a directional blurring of vertical lines. As ICF stack is placed with long edge parallel to the vertical lines, its definitive proof of vertical burring function of the OLPF layer.
Left: with OLPF; Right: without. The distance between 2 lines in a square is 50um.
Clear glass replacement
Now in order to achieve infinity focus for my 18-105 lens, I’m going to replace the ICF with a clear optical glass. One way to test if the optical depth differences is simply to use a microscope. Place a sheet of paper underneath the filter and focus the microscope at 200x on the thin fiber on the paper. Then gentle replace it with the new optical glass. If the focus is sharp, then at least this means the focus remained the similar. The rest of the error will be within the range of D7000’s focus fine tune system.
The replacement HK-9 glass with a APS-C sensor as size comparison
Notice the bonding wire is sufficiently different from IMX038, where a lot of camera website used the wrong photo for this sensor. (Click for large image)
The procedure is the same, just place the glass in the same rubber gasket frame using forceps.
The rubber gasket sits on the IMX071 Sony CMOS sensor
This replacement glass lacks anti-reflection coating. The thickness is 40.3 Mil or 1.023mm. HK-9 glass is equivalent to BK-7 borosilicate glass with a refractive index of 1.5. After replacement, The focus almost comes back to the original factory setting.
Transmission Analysis
To roughly assess the transmission, I used the same lens setting to infinity focus and take a RAW image of 1/5s exposure at the LCD screen with the same aperture, ISO and the maximum brightness setting of LCD panel. The table below shows the raw average ADU count in the center uniformed area of image, and the ratio is normalized to ICF removal one.
|
|
RAW ADU
|
Ratio
|
|
|
R
|
G
|
B
|
R
|
G
|
B
|
|
Before
|
5738
|
11257
|
7947
|
0.51
|
0.86
|
0.92
|
|
ICF Remove
|
11308
|
13114
|
8643
|
1.00
|
1.00
|
1.00
|
|
HK-9
|
10651
|
12312
|
7966
|
0.94
|
0.94
|
0.92
|
It is astonishing that we have almost 1 fold gain in the red channel. The green channel is also improved a little. Note that HK-9 reflect and absorb 6-8% light in all channel.
Color Correction Filter Spectrum Property
Since gaining 1 fold more red light in a channel doesn’t mean the distribution of that gain is equal at different wavelength within. Here we use a UV-VIS spectrophotometer to measure the transmission profile of the infrared absorbing glass.
Transmission property of ICF, replacement glass and a quartz glass window (RCP) from 200 to 900nm. Actually I doubt the replacement glass as HK-9 will strongly absorb at 300nm, and the transmission curve closely resembles N-BK7 glass. Also notice that an uncoated glass window will reflect 8% of light in total on 2 interfaces. The quartz filter is broadband coated, which bring the transmittance up to >96%. The quartz is transparent down to 160~180nm in UV below the detection range of this spectrophotometer.
After all, this means we had a 4 fold gain at 656nm Ha emission line plus an increased QE of new generation sensor, personally I’m well satisfied with this result.
Updated 1/22/2013
