The Truth about Neutrality

Transcription

1 The Truth about Neutrality How Close is Close Enough By Ron Engvaldsen Schneider Optics Inc. March 23, 2017 For many years, the manufacture of neutral density filters (ND s) has been a challenge for all filter optical companies. Too cool? Too warm? Too magenta or green? Too dense or not dense enough? With over 20 years of experience testing ND s from most all manufacturers, I believe I can speak on behalf of anyone that has or will attempt to manufacture the Perfect neutral gray in saying, Perfect is and will continue to be an elusive achievement. So, if perfect neutral gray is not achievable, some questions must be asked fundamental as well as advanced: Fundamental: How are ND filters currently measured for neutrality? If the measured filter is not perfectly neutral, how close is close enough? Page 1 of 14

2 Advanced: How should ND filters be measured for neutrality? How can we establish a method for strict quality control? Before these questions can be answered, let s touch on some basic theory. How is neutrality defined? The definition of neutrality is The spectral equality of red, green & blue. This is overly fundamentalized however and for purposes of understanding the definition as it applies specifically to this paper, and with a bit of rewording, it is more appropriate to define neutrality as, The spectral equality of all values of transmission for each wavelength within a prescribed range of visible light. For many years, filter manufacturers have utilized a single method for the determination of neutrality Spectrophotometry. A spectrophotometer is an instrument used to scan the transmission of light through a sample (filter) across a selected wavelength range. The desired result of a spectrogram for the perfect neutral is for the % Transmission (%T) to be the same within a 400nm to 800nm* wavelength region. Note: In theory, the visible spectrum of light encompasses the wavelength range from 400nm to 750nm. * 1 nanometer (nm) = 1x10-9 m Page 2 of 14

3 Below is a sample of a spectrogram (transmission plot) taken through a random ND filter: You can see the %T (y-axis) is approximately the same across the wavelength range (x-axis.) The operative word in the previous statement is approximately. I ll get back to this. Page 3 of 14

4 So, back on page 2-3 there were some unanswered questions: Fundamental: How are ND filters currently measured for neutrality? Answer: The sample filter is scanned on a spectrophotometer. An evaluation is made to determine how flat the spectrogram (curve) is. If the measured filter is not perfectly neutral, how close is close enough? Answer: A range of acceptability (+/- of %T) is established. It is normally a percentage of the theoretical transmission. Example as follows with a lower/upper limit set to 10/12 %T: Page 4 of 14

5 Advanced: How should ND filters be measured for neutrality? Answer: o Scan the sample on the spectrophometer from nm with light source set to D6500. * o Export %T data in 5nm intervals into a tristimulus ** algorithm and record x, y tristimulus coordinate. o Plot the x, y tristimulus coordinate on CIE 1931 Chromaticity Diagram. o Compare to coordinate for D6500 (.313,.329.) o Sample plotting same as D6500 is perfect neutral. * D6500 = Standard Daylight Illuminant: CIE Standard Illuminant D65 (sometimes written D 65 ) is a commonly used standard illuminant defined by the International Commission on Illumination (CIE). It is part of the D series of illuminants that try to portray standard illumination conditions at open-air in different parts of the world. ** Tristimulus defined: of or relating to values giving the amounts of the three colored lights red, green, and blue that when combined additively produce a match for the color being considered. Page 5 of 14

6 Below is the CIE 1931 Chromaticity Diagram showing the D65 point at about 11 O clock on the white (neutral) zone x, y coordinate = (.313,.329) Page 6 of 14

7 Below you can see the D6500 point and the ND (sample) point plotted on an x, y graph. If you observe the range & domain of the graph below you can see how this graph represents a small, central section of the CIE 1931 diagram. Coordinates: x y D ND Sample Page 7 of 14

8 You can see the sample plot is fairly close to the D6500 point. How close is close enough? How can we establish a method for strict quality control? Answer: o Establish a MacAdam Ellipse * a boundary which contains the data point for D6500. This boundary acts a go-nogo gauge for whether a sample falls within the ellipse and meets the prescribed specification for neutrality. Ordinarily, the central locus of the ellipse is considered the nominal point but in the example shown below, the nominal point is actually off-center. The reason for this is due to there being a reduced tolerance towards the 10 O clock direction and greater tolerance in the 4 O clock direction. Thus, we can describe this ellipse as a Modified MacAdam Ellipse. o If the sample plots outside the ellipse, it could be suspect of resulting in an unacceptable color shift. o While sometimes not practical, the best method for determining the limits of the ellipse boundary is to establish a series of data points (colors) that will reside outside the ellipse based on unacceptable color shift. * MacAdam Ellipse defined: In the study of color vision, a MacAdam ellipse is a region on a chromaticity diagram which contains all colors which are indistinguishable, to the average human eye, from the color at the center of the ellipse. Page 8 of 14

9 The following is an example of an x,y plot of a sample ND, D6500 both contained in a Modified MacAdam Ellipse. Page 9 of 14

10 Why is tristimulus calculation the best method for the true determination of neutrality? Let s look back at the graph on pg. 5: As stated earlier, the values of %T are approximately the same across the spectrogram. If one wavelength region exhibits a higher %T than another, even though the curve seems relatively flat, this can result in an uneven spectral response. This imbalance can cause a lack of neutrality. Therefore, establishing a +/- tolerance on %T values across the visible range can lead to the appearance of neutrality or a flattish curve based but this can result in a color biased neutral. Page 10 of 14

11 Another Method of Determining Neutrality Camera rental facilities do not have access to a spectrophotometer nor the time it takes to determine an ND s level of neutrality. A quick and easy way to determine a filter s neutrality is by testing the sample on the very cameras on which the filters will be used. The output of the camera can be piped out to a Vectorscope. Here is an example of a vectorscope display: There are 6 sectors of the 360 degree display, each one representing either a primary or secondary color. Below is an image showing the colors: Page 11 of 14

12 The camera is focused on an optical target that represents a neutral gray. With no filter in front of the lens, the vectorscope will display a burst in the center of the display the center is considered perfectly neutral as it represents an equal response from all 6 colors. With a filter in front of the lens, the center burst should remain in the center. A shift into any of the six sectors indicates a color shift of that sector. Note: The relationship between the magnitude of the shift and whether that shift would actually be noticed in a real image on camera, is difficult to quantify. However, as empirically tested, if the vectorscope is put on a 10X (times ten) range, a small shift (nudge) of the center burst can be considered a fine neutral. Another important factor about neutral density testing is what is known as End-to-End testing. Vectorscope testing is very effective because it is a test of the image through the filter lens sensor electronics monitor. Page 12 of 14

13 Summary: Over the years, I ve been told by many Directors of Photography, camera assistants etc. that there is great expense in post-production correction caused specifically from A & B cameras using mismatched ND s. Newer and better ND filters require better methods of testing to determine their true neutrality. It is incumbent upon filter manufacturers that the proper method of testing and evaluation be employed to ensure that ND filters meet the standards expected by the professionals & amateurs utilizing our products. Will all filter manufacturers represent spectrographic curves to show the actual error? Answer: Not always. I have seen representations of neutral density filters where the curves were presented with scaling that buries the perception of the error. Is this intended? Maybe. I ll refer back to the same spectrogram from page 5. You ll notice I scaled this curve from 0 to 20% transmission. This shows the real error in the curve. Page 13 of 14

14 Here s the same curve scaled from 0 to 100% T: You can see how much flatter the curve appears simulating equal spectral response across the visible region. I think I ve shown how spectrograms alone cannot determine the Truth about Neutrality. However, the acquisition of spectral data is of paramount importance in the determination of the proper data points that land inside or outside of the ellipse and help us to determine, How Close is Close Enough. R.E. Page 14 of 14