diff --git a/hsl.mbt b/hsl.mbt
new file mode 100644
index 0000000..49061c2
--- /dev/null
+++ b/hsl.mbt
@@ -0,0 +1,98 @@
+///|
+/// HSL color space: the "lightness" cousin of HSV. Hue is shared with HSV;
+/// lightness places white at L=1 and black at L=0 with pure colors at
+/// L=0.5, which makes symmetric lighten/darken operations natural. The
+/// conversions mirror the existing HSV implementations so 8-bit round
+/// trips are exact.
+
+///|
+/// Converts 8-bit RGB to HSL: hue in degrees `[0, 360)`, saturation and
+/// lightness in `[0, 1]`.
+pub fn rgb_to_hsl(r : Int, g : Int, b : Int) -> (Double, Double, Double) {
+  let rf = r.to_double() / 255.0
+  let gf = g.to_double() / 255.0
+  let bf = b.to_double() / 255.0
+  let mut mx = rf
+  if gf > mx {
+    mx = gf
+  }
+  if bf > mx {
+    mx = bf
+  }
+  let mut mn = rf
+  if gf < mn {
+    mn = gf
+  }
+  if bf < mn {
+    mn = bf
+  }
+  let d = mx - mn
+  let l = (mx + mn) / 2.0
+  let h = if d == 0.0 {
+    0.0
+  } else if mx == rf {
+    wrap_hue(60.0 * (gf - bf) / d)
+  } else if mx == gf {
+    60.0 * ((bf - rf) / d + 2.0)
+  } else {
+    60.0 * ((rf - gf) / d + 4.0)
+  }
+  let s = if d == 0.0 {
+    0.0
+  } else if l < 0.5 {
+    d / (mx + mn)
+  } else {
+    d / (2.0 - mx - mn)
+  }
+  (h, s, l)
+}
+
+///|
+/// Converts HSL (hue in degrees, saturation/lightness in `[0, 1]`) back to
+/// 8-bit RGB using the standard chroma/sector decomposition.
+pub fn hsl_to_rgb(h : Double, s : Double, l : Double) -> (Int, Int, Int) {
+  let ld = if l - 0.5 < 0.0 { 0.5 - l } else { l - 0.5 }
+  let c = (1.0 - 2.0 * ld) * s
+  let hh = wrap_hue(h) / 60.0
+  let mod2 = hh - 2.0 * ifloor(hh / 2.0).to_double()
+  let diff = if mod2 - 1.0 < 0.0 { 1.0 - mod2 } else { mod2 - 1.0 }
+  let x = c * (1.0 - diff)
+  let sector = ifloor(hh)
+  let (r1, g1, b1) = match sector {
+    0 => (c, x, 0.0)
+    1 => (x, c, 0.0)
+    2 => (0.0, c, x)
+    3 => (0.0, x, c)
+    4 => (x, 0.0, c)
+    _ => (c, 0.0, x)
+  }
+  let m = l - c / 2.0
+  (
+    ((r1 + m) * 255.0 + 0.5).to_int(),
+    ((g1 + m) * 255.0 + 0.5).to_int(),
+    ((b1 + m) * 255.0 + 0.5).to_int(),
+  )
+}
+
+///|
+/// Shifts every pixel's HSL lightness by `delta` (in `[-1, 1]`), clamping
+/// the result into `[0, 1]`. Positive values lighten toward white while
+/// preserving hue; negative values darken toward black. `delta == 0`
+/// returns a copy.
+pub fn Image::adjust_lightness(self : Image, delta : Double) -> Image {
+  if delta == 0.0 {
+    return self.copy()
+  }
+  self.map_rgb(fn(r, g, b) {
+    let (h, s, l) = rgb_to_hsl(r, g, b)
+    let nl_raw = l + delta
+    let nl = if nl_raw < 0.0 {
+      0.0
+    } else if nl_raw > 1.0 {
+      1.0
+    } else {
+      nl_raw
+    }
+    hsl_to_rgb(h, s, nl)
+  })
+}
diff --git a/hsl_test.mbt b/hsl_test.mbt
new file mode 100644
index 0000000..233c542
--- /dev/null
+++ b/hsl_test.mbt
@@ -0,0 +1,87 @@
+///|
+/// HSL tests: primary-color reference values, 8-bit round trips, gray
+/// invariance and lightness adjustment behavior.
+
+///|
+test "rgb_to_hsl on primaries and white matches reference values" {
+  // Pure red: H=0, S=1, L=0.5.
+  let (h, s, l) = rgb_to_hsl(255, 0, 0)
+  assert_eq(h.to_int(), 0)
+  assert_eq((s * 100.0).to_int(), 100)
+  assert_eq((l * 100.0).to_int(), 50)
+  // Pure green / blue hues.
+  assert_eq(rgb_to_hsl(0, 255, 0).0.to_int(), 120)
+  assert_eq(rgb_to_hsl(0, 0, 255).0.to_int(), 240)
+  // White: L=1, S=0.
+  let (_, sw, lw) = rgb_to_hsl(255, 255, 255)
+  assert_eq((sw * 100.0).to_int(), 0)
+  assert_eq((lw * 100.0).to_int(), 100)
+}
+
+///|
+test "hsl round-trips 8-bit colors exactly" {
+  let samples = [
+    (255, 0, 0),
+    (0, 255, 0),
+    (0, 0, 255),
+    (255, 255, 0),
+    (12, 200, 99),
+    (128, 128, 128),
+    (0, 0, 0),
+    (255, 255, 255),
+    (37, 142, 209),
+    (250, 128, 114),
+  ]
+  for i in 0..<samples.length() {
+    let (r, g, b) = samples[i]
+    let (h, s, l) = rgb_to_hsl(r, g, b)
+    let (r2, g2, b2) = hsl_to_rgb(h, s, l)
+    assert_eq(r2, r)
+    assert_eq(g2, g)
+    assert_eq(b2, b)
+  }
+}
+
+///|
+test "grays keep zero saturation and exact lightness" {
+  for v in [0, 64, 128, 200, 255] {
+    let (_, s, l) = rgb_to_hsl(v, v, v)
+    assert_eq((s * 1000.0).to_int(), 0)
+    // L = v/255 exactly.
+    let (r2, g2, b2) = hsl_to_rgb(0.0, s, l)
+    assert_eq(r2, v)
+    assert_eq(g2, v)
+    assert_eq(b2, v)
+  }
+}
+
+///|
+test "adjust_lightness lightens toward white and darkens toward black" {
+  let img = single(200, 50, 50, 255)
+  // +1 lightness saturates to pure white regardless of hue.
+  let white = img.adjust_lightness(1.0).get_pixel(0, 0)
+  assert_eq(white.0.to_int(), 255)
+  assert_eq(white.1.to_int(), 255)
+  assert_eq(white.2.to_int(), 255)
+  // -1 lightness collapses to black.
+  let black = img.adjust_lightness(-1.0).get_pixel(0, 0)
+  assert_eq(black.0.to_int(), 0)
+  assert_eq(black.1.to_int(), 0)
+  assert_eq(black.2.to_int(), 0)
+  // A small positive delta brightens every channel without reordering.
+  let (r, g, b, _) = img.adjust_lightness(0.2).get_pixel(0, 0)
+  assert_true(r.to_int() > 200)
+  assert_true(g.to_int() > 50)
+  assert_true(r.to_int() > g.to_int())
+  assert_eq(g.to_int(), b.to_int()) // symmetric channels stay symmetric
+}
+
+///|
+test "adjust_lightness(0) is an exact copy and alpha is preserved" {
+  let img = single(120, 88, 33, 77)
+  let same = img.adjust_lightness(0.0)
+  for i in 0..<4 {
+    assert_eq(same.data[i].to_int(), img.data[i].to_int())
+  }
+  assert_eq(img.adjust_lightness(0.3).get_pixel(0, 0).3.to_int(), 77)
+}
diff --git a/pkg.generated.mbti b/pkg.generated.mbti
index 2f63eec..59c44b7 100644
--- a/pkg.generated.mbti
+++ b/pkg.generated.mbti
@@ -12,6 +12,8 @@ pub fn gif_decode(Array[Byte]) -> Image?
 
 pub fn hamming_distance(UInt64, UInt64) -> Int
 
+pub fn hsl_to_rgb(Double, Double, Double) -> (Int, Int, Int)
+
 pub fn hsv_to_rgb(Double, Double, Double) -> (Int, Int, Int)
 
 pub fn png_decode(Array[Byte]) -> Image?
@@ -22,6 +24,8 @@ pub fn qoi_decode(Array[Byte]) -> Image?
 
 pub fn qoi_encode(Image) -> Array[Byte]
 
+pub fn rgb_to_hsl(Int, Int, Int) -> (Double, Double, Double)
+
 pub fn rgb_to_hsv(Int, Int, Int) -> (Double, Double, Double)
 
 pub fn rgb_to_ycbcr(Int, Int, Int) -> (Int, Int, Int)
@@ -66,6 +70,7 @@ pub(all) struct Image {
 }
 pub fn Image::add_gaussian_noise(Self, UInt64, Double) -> Self
 pub fn Image::add_salt_pepper(Self, UInt64, Double) -> Self
+pub fn Image::adjust_lightness(Self, Double) -> Self
 pub fn Image::affine(Self, Affine) -> Self
 pub fn Image::apply_filter_id(Self, Int, Double) -> Self
 pub fn Image::auto_contrast(Self) -> Self