Color Control of Cell Phone Housings

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Color Control of Cell Phone Housings

• Konica Minolta Sensing, Inc.

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Color control of cell phone housings

• Set color standards for each part, since even if
  the same paint is used, the painted color may be
  different.
• Set different control tolerances according to how
  noticeable the part is and the manufacturing
  conditions for the part.
• Use trend graphs to control color shifts between
  lots.

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Set color standards for each part,

• since even if the same paint is used, the painted
  color may be different.

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Set color standards for each part, since even if
the same paint is used, the painted color may be
different.

• Cell phone housings are made up of multiple parts

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Set color standards for each part, since even if
the same paint is used, the painted color may be
different.

• Even if the same paint is used, depending on part
  size/shape, painting conditions (painting speed,
  number of coats) may be different, causing
  differences in paint layer thickness, glitter
  material distribution, etc. The result is slight
  differences in measured values.

L32.47 a45.44 b21.29
L31.65 a44.97 b20.80
Even though the paint is the same, measured
values are completely different
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Set color standards for each part, since even if
the same paint is used, the painted color may be
different.

• Measurement of parts which are rounded or which
  become distorted under the force applied by the
  measuring instrument require care when measuring,
  since there is a gap between the part surface and
  the instrument's measuring aperture, resulting in
  values which are different than those which would
  be measured if the surface was flat.
• Care must also be taken when measuring parts
  which are smaller than the instrument's measuring
  aperture.

Rounded parts
Distorted parts
Small parts
Measuring aperture
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Set color standards for each part, since even if
the same paint is used, the painted color may be
different.

• Rounded parts can be measured if the same
  measurement point is always used and measurements
  are taken the same way.

First measurement
First measurement
Second measurement
Second measurement
OK
NG
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Set color standards for each part, since even if
the same paint is used, the painted color may be
different.

• Parts which may be distorted can be measured if
  the force applied to the part by the measuring
  instrument is kept uniform, such as by inverting
  the instrument so the measuring aperture faces
  upward and placing the part on top of the
  instrument.

First measurement
First measurement
Second measurement
Second measurement
OK
NG
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Set color standards for each part, since even if
the same paint is used, the painted color may be
different.

• Parts which are smaller than the instrument's
  measuring area can be measured if the relative
  positions of the aperture and part are kept
  constant and the area within the measuring
  aperture but not covered by the part is kept
  uniform. Color control is relative to the target
  color.

First measurement
First measurement
Second measurement
Second measurement
OK
NG
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Set color standards for each part, since even if
the same paint is used, the painted color may be
different.

• Since painting conditions and measurement
  conditions are different for each part, it is
  necessary to select one of each type of part as
  the standard part for that type, measure it, and
  set the measurement results as the standard color
  for that part type.

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Set color standards for each part, since even if
the same paint is used, the painted color may be
different.

• Since the shape of each part is different, if the
  same position isn't measured every time,
  measurement results will vary. Attach a photo
  specifying the measurement point for each part.

You can add a marker to indicate the measurement
point.
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Set different control tolerances

• according to how noticeable the part is and the
  manufacturing conditions for the part.

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Set different control tolerances according to how
noticeable the part is and the manufacturing
conditions for the part.

• When a product is assembled, there are parts
  which are very noticeable and parts which are
  less noticeable. Because of this, although tight
  tolerances should be set for very noticeable
  parts, setting looser tolerances for parts which
  are less noticeable is acceptable.

Very noticeable part
Less noticeable part
Tolerances DL1.0 Da0.3 Db0.5
Tolerances DL1.4 Da0.5 Db0.7
Note Tight and loose tolerances should differ up
to around 0.5 in terms of DEab.
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Set different control tolerances according to how
noticeable the part is and the manufacturing
conditions for the part.

• Depending on part characteristics such as shape,
  etc., there are cases where it is difficult to
  paint a part in the same way as for other parts,
  making it difficult to control its color using a
  standard part.
• In such case, feasible actual control tolerances
  should be provided to the customer beforehand for
  approval.

Standard part
Part which is difficult to paint
Tolerances DL1.0 Da0.3 Db0.5
Tolerances DL1.4 Da0.5 Db0.7
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Use DE94 for controlling colored parts

• Color control by DEab results in a spherical
  region, and DL, Da, Db results in a cubic
  region, but color difference judgment by human
  eye results in ellipsoidal regions in color
  spaces. Because of this, DE94, in which color
  control is performed using ellipsoidal regions,
  is very useful.
• Setting the control tolerances should be
  performed by measuring past acceptable and
  unacceptable parts, and creating the ellipsoid
  accordingly.

Details
Setting the ellipsoid size
lLightness-direction shape cSaturation-direction
shape hHue-direction shape
Setting DE94 control tolerances
DE94 control tolerances and distribution of
measurements
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Use DL, Da, Db for controlling parts colored
silver or similar colors

• The color of parts colored silver or similar
  colors differ mainly in lightness, and do not
  differ much in color.
• Because of this, looser tolerances should be set
  for lightness, and tight tolerances for color.
• An actual example is shown below.

Silver-colored parts
Tolerances DL1.5 to 2.2 Da0.3 Db0.5
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Use trend graphs

• to control color shifts between lots.

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Use trend graphs to control color shifts between
lots.

• During assembly, when the quantity of parts
  becomes low, the parts from a lot received early
  may be mixed with parts from lot received later.

Mixed here
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Use trend graphs to control color shifts between
lots.

• Although both lots look OK compared to the upper
  and lower limit samples, in some cases the color
  differences may be remarkable and can cause
  problems

Upper/ lower limit samples
When lots are mixed...
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Use trend graphs to control color shifts between
lots.

• Trend graphs are useful for controlling color
  shifts between lots and eliminating the problem
  of color shifts between lots.

1st lot
4th lot
3rd lot
2nd lot
1st lot
4th lot
3rd lot
2nd lot
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Use trend graphs to control color shifts between
lots.

• Attaching a trend graph to the delivery report
  makes judgment easy.

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Recommended screen layout for color control of
cell phone housings

• Shown in SpectraMagic NX

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Recommended screen layout for color control of
cell phone housings (Shown in SpectraMagic NX)
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Other points requiring attention

• (2 points)

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Other points requiring attention1. Be careful of
metamerism in parts using different paints.

• Different paint makers use different paints, so
  care must be taken regarding metamerism.
• Metamerism is the phenomenon that objects which
  appeared to have the same color under one light
  source (the sun, for example) appear to have
  different colors under another light source
  (fluorescent lamps, for example).

Standard plate
Sample
Standard plate
Sample
Same color
Different colors
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1/6
Spectral response of human eye (Tristimulus
values)
?
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2/6
???
Spectral response of human eye (Tristimulus
values)
??
?
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3/6
???
???
Same amount
Same amount
?Eab0
Same amount
Same color
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4/6
???
Spectral response of human eye (Tristimulus
values)
??
?
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5/6
???
Spectral response of human eye (Tristimulus
values)
??
?
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6/6
???
???
Different amount
Different amount
?Eab1.5
Different amount
Different color
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Other points requiring attention2. Metallic
paints appear different at different viewing
angles

• Metallic paints have aluminum or mica flakes
  (minute thin chips) mixed in.
• These flakes face the surface at almost the same
  angle, but there is sufficient variation that
  specular light is reflected at a variety of angles

Light reflected specularly from flake surfaces
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Other points requiring attention2. Metallic
paints appear different at different viewing
angles
Light component specularly reflected from paint
layer
Light component specularly reflected from
surfaces of flakes
Incident light
Diffusely reflected light component

• Since the amount of specularly reflected light is
  different depending on the angle, it also appears
  very different to the human eye. This is called
  "flip-flop"

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Other points requiring attention2. Metallic
paints appear different at different viewing
angles

• For color control (flip-flop control) of metallic
  paints, evaluation from multiple directions is
  good. The directions should include the highlight
  direction (with the influence of specularly
  reflected light) and the shade direction (without
  the influence of specularly reflected light).
• Since different paint makers offer different
  paints, flip-flop control is necessary.

Multi-direction ring illumination
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